Hello GOG members
First of all I'd like to say that I'm somewhat new here, and if there's something like a welcome thread, I'd happily make an introduction post there.
Now for the actual question, I've quickly checked the FAQ and haven't found anything here, so thought I'd ask here:
What are the requirements for getting a game on the GOG Store?
I'm not talking about buying games here, but about(for example) a developer that has a game and would like to put it on GOG.

I hope to do more with this community as GOG is a very nice thing.

GOG has nothing like Steam Greenlight where your rpg maker/unity crap can make it onto the platform. We already have developers part of this community whom GOG doesn't think are worth supporting. If their curation manager doesn't think it's the next best thing to sliced bread expect the "it's too niche" excuse for rejecting it. You'll be in the company of many good developers when you get that one.

First of all, welcome.
Second, I assume the FAQ you read is the one in this page. If not, this is the one to read.
Now, getting a game on GOG is not that easy. First big suggestion is to send GOG a final build of your project, not the early prototype. GOG has the tendency to reject alpha builds, though they may take a second look once the game is complete.
Second suggestion is to know that GOG does take its time to reply. There doesn't seem to be a "We've received your submission and are looking into it" email, so developers are often wondering what is happening for a few weeks while GOG looks over the game.
Third point, GOG does reject quite a lot of games, even if they are from a genre that should be welcomed here. What makes your game unique? Why should one pick your game over any other from the same genre? Is your unique twist actually unique, or is it just a different paint (so to speak)?
Another point that I assume (but don't have any actual evidence) is the price point of the game. No idea if GOG still makes suggestions on what they think would be a good price point or not, but I think it's possible they may reject a game as too pricey.

Final two pieces of advice. First, hype your game. Make a thread in the forum about your game, along with a wishlist entry, then talk to us and tell us why we should be interested in your game. Do a developer's diary, so to speak, similar to what HiddenAsbestos did with Grid Cartographer. It may not have been a game, but it did get quite a bit of support in the forum.
Second, should you submit your game and get the "Too niche" reply, do not be disheartened. This seems to be the standard "We don't like the game" reply, but GOG has been known to change their minds about games, and take them on at a later point.

You may also want to ask some of the indie developers that actually got here, like Lars Doucet of Level Up Labs. NDAs may apply though, so not sure how much they can help you.

Either way, I do wish you good luck with your game, and hopefully we will see it on GOG.

I do hope that good RPG Maker/Unity/Gamemaker stuff is allowed, while I can definitely see why one would say "no such things allowed here", it can be quite sad for some developers who actually made something fun, the big problem here is that those tools easily give everyone access to creating own games, and sadly not everyone thinks about what they're doing and if the final product is actually enjoyable. Alright I see, guess those points are a bit "softer" with bigger game developers and mostly there to keep the "generic" stuff away, basically if someone made a game with one of those * makers that isn't terrible but not great as well.
Does GOG accept free games, like if the developer wants to release the game completely free(not free to play, completely free)?
I currently don't have a game(but I do plan to work on one in the future, do already have plenty of coding knowledge), was more curious on the more technical details, like what's required for the Galaxy Client, Price to enter and all that.

They do, but again the question is "why should they do that"? Even a free game does cost GOG time and money for testing, packaging and hosting, so the game does have to be exceptional for them to do so. Of the current free games on GOG, only two are recent-ish, Treasure Adventure Game released on 2011 and Warsow released on 2012. The other games are older ones that have had their status changed to freeware. No idea on these two points. I'd think that GOG would prefer it if you did integrate Galaxy features into your game (achievements for example), but the community would prefer it if you didn't need them (galaxy free multiplayer for example).
Price is also a potential roadblock, since you must make sure the game is not too expensive, but it also has a price you consider fair. I think they do give a bit of advice on that, but not sure they do.