Why don't we get a central location for all this stuff?
If an issue seems to keep popping up I will do my best to add it to the faq for others. Unfortunately I can't provide technical support, sorry.

Acronyms used:
AvP - Aliens versus Predator
GoGaxy - GoG Galaxy (I'm lazy :P)

Installing:
-So to start.. download the game, yay!
-Install the base AvP game where you want.
-The GoGaxy client will default to [ C:\Program Files (x86)\GalaxyClient ] on 64 bit systems. Seems this is not changeable currently. GoGaxy is included with the base game.

Running:
-For single player you can use [AvP install dir\]AvP_Classic.exe. (AvP Classic)
-For multiplayer you need to run the game via the Galaxy client¹. There is a shortcut in your start menu (Aliens vs Predator Classic 2000 - Galaxy Client).
-Galaxy will install an update the first time, and periodically as needed.
-Your firewall should ask permission to allow GoGaxy internet permissions, do so. If not you will have to do so manually, check its guides or google for this if unsure.
-Launch the game from the client and select multiplayer, give it a few seconds to load the game list.
-Select a game to join, select your race/class, start killing!

FAQ!
Q: Why is the game not showing up?
Ensure you added the key to your account via the link in the email or manually here, and that it was the current account logged in with.
Next up you can try an account refresh, https://www.gog.com/account/refresh

Q: Where is GoGaxy/how do I download it/where did it install?
Just in case you skipped to the faq:
-The GoGaxy client will default to [ C:\Program Files (x86)\GalaxyClient ] on 64 bit systems.
-Seems this is not changeable currently.
-GoGaxy is included with the base game.

Q: Failed to initialise, can't play multiplayer?
Please ensure you are running the game from the GoGaxy client. (GoGaxy Client)

Q: Joining a game takes too long or errors out?
This seems to be a hiccup with the online system or the game filled up before you were able to connect. Try checking the multiplayer list again and rejoining or select another match.

Q: Why is the server list is taking longer to load?
Give it some time. This generally happens if you cancel joining a game then go back to multiplayer.

Q: The game is super fast! Can anything be done to slow it down?
Yep, welcome to old school shooters. Try only joining or creating servers with a reduced game speed. The game speed is shown when joining a game. (Game Speed)

Q: Why is the game screen so tiny, changing the setting doesn't work?
The initial menus seem to stay the same size regardless of the chosen resolution but that's ok. When you play single or multiplayer the size will be what you set it to.

Q: How do I change screen resolution?
Menu -> Audio/Video Options -> Video Card & Resolution -> Press Enter on the size in order to change the setting. (Screen resolution, graphical version.)
Note the main menu will not change size.

Q: Why is the game so dark?
It's all moody and spooky lighting. Try increasing the gamma within the game options. Also don't forget about night vision and flares.
While playing the game, single or multiplayer:
-While PLAYING pull up the menu, select Audio/Video Options (Step 1)
-Gamma settings (Step 2)

Q: No seriously why is the game pitch black?
Looks like someone destroyed the lights in the level...

Q: Looking around seems slow?
For the speed this game plays at you may want to increase the mouse sensitivity in game. May be good to keep the horizontal and vertical rates the same. Around 60% on the bar seems fairly responsive.
Menu -> Controls -> Mouse Configuration -> Vertical/Horizontal Sensitivity (Mouse Seneitivity, graphical version.)

Q: How do I walk on walls and ceilings as an Alien?
Hold crouch, default button for that is left Ctrl.

Q: Is there in game chat?
Yus! It is located at the top center where death messages are. Default keys are T for all chat and Y for team chat. (Chat)

Q: Help, can't close the chat window!
Delete everything in the chat window and type in "SAY " sans the quotes. Hit enter and no more chat window.

Q: Someone is griefing me how do I stop them/report them?
Unfortunately it seems there are no options to ban or kick people. Not sure how to make private games either, anyone know?
Perhaps GoGaxy will give some of these options when it is fully released.

Q: Is it possible to stop griefing in coop?
Ensure friendly fire is off before hosting or joining a game.
The host could try setting the game up with limited slots however that would only be useful for pre-organised matches.

Q: Why can't I see my ping?
Unfortunately the game doesn't show your own ping.

Q: Why does my ping seems really high?
Most random games and when you make a match use that users machine to host the game. Where they are and their internet speed will greatly affect pings.

Q: Why does the game seems inbalanced, uneven, unfair with some weapons?
It really is. The main way to balance it is how a host sets the match up.
Try to join games with disabled weapons; Predator: Disk/Pistol, Human:Grenade Launcher/Skeeter

Q: What is "This machine is now host"?
This will pop up in the chat area when the original host of the game disconnects. This means that you are now hosting the match. Amazing that dynamic rehosting existed back then!

Q: Can the game be played in windowed mode?
Yes, check here: http://www.gog.com/forum/aliens_versus_predator_classic_2000/run_windowed

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¹Here is the shortcut just in case: For example, this is my full shortcut:

Thanks this was really helpful actually.

Does the in game chat work? I wasnt getting any responses after playing in several different sessions.

It is working, but i suppose few players have time to reply if they can even see your chat. But it does work. :)

Yep it works, people may not notice it since it's at the top of the screen. Also doesn't stick around long especially when lots of killing scrolls it.

Hello,

I have a Problem with the Sound/Music in the Game. I hear nothing. How can I fix this?

Everything works fine for me, but mouse seems to have some kind of acceleration and making snipe shots on extreme range very difficult. Is there a way to disable it in some config files or something like that?

Sorry guys, no idea.
Best bet is making a separate post to check if anyone else has had a similar issue. Will try my best to add any frequent issues to the faq.

Made a test video in AvP to see how well it works. Seems to do rather well though was thinking this game could make people rather nauseous. Instead of making a new topic for this mineaswell put it here as a semi-excuse to bump ^.^

https://www.youtube.com/watch?v=SlGWKztYmy0

I was amused at the suicide into the electric fence. That damn thing always gets me when I least expect it.

Just joined last night. I love this game - always did. I liked AVP2 even maybe a little better but this is great. So fast though!

Was thinking it would be neat to have a tournament of some sort.
Not sure how to handle it all though since the game really is not set up to support it. Anyone have any thoughts on this? At least there's no comfy chair. That would just be pure torture for an Alien.

The game is set up to support it same as any other game.

GOG, on the other hand, needs to have its community aspect more developed so that players could interact and coordinate better outside of the game itself.

On Steam we already have an organized group dedicated to community events and tournaments.

Our site can be found here: http://avpct.com/

Once GOG enables cross-play, you're more than welcome to join us.

There's no invis/noclip or observer mode and hosting options are rather limited. Still possible to do though was not thinking of 1v1 style.

Those options could definitely improve the multiplayer experience, but still, they are not a mandatory feature for hosting tournaments - even if you're aiming at team games in an SDM.

2vs2 and 3vs3 games (4vs4, I think, would be too chaotic for this game) can be easily coordinated through chat once all the players join the game.

One thing the GOG version does lack and is much more important than invis/noclip or observer mode in my opinion, is the ability to host private games; Something the Steam version does allow you to do (makes one wonder why it was removed with the release of the GOG version).

The way GOG is currently set up, the "host private game" function can't work. On Steam, one can only connect to a private game through one's friend list or invite function. It works in tandem with the third-party client itself. Because GOG doesn't have that community aspect built up (for instance, there is no friend feature yet), it wouldn't make sense to make a private matches option. Once the Galaxy client is developed, then maybe it has the potential for private matches to work.