My roommate and I were debating which version to get. Steam or GOG for NMS. I was leaning GOG and he was leaning steam.

One point we were trying to clear up. NMS can be played completely offline but what about online? If GOG is DRM free how does online work for NMS for Online usage/play for this game?

Unlike Elite Dangerous No Man's Sky uses mathematics function for procedural generating worlds and don't depend on online servers.
Online functions (like sharing your discovered world and multiplayer mode) will available when you will be online.
If you want to have game without DRM GOG is ONLY option. :)

I don't know about NMS in particular, however in general GOG's DRM-free extends only to single player experience i.e. if games have an additional multiplayer/online feature it might require you to create an additional online account for the game. Victor Vran is an example.

That said, I'd if there is no reason to pre-order the game, I suggest you simply wait till it's out. Then you'll know for sure how online play for this particular game is handled by GOG.

It's possible NMS will use Galaxy for online on GOG and will use Steamworks on Steam to offer online, or dev may have made their own service to handle this. The ability to play together or not on both platforms is up to the dev.

But honestly it doesn't really matter with this game, dev has said it's highly unlikely you will ever run into anyone online because the world is so big and I don't think you can join in with friends or anything with this game. It's actually meant to be more of single player experience.

You might want to have a look at this reddit. It's very informative. Regarding the online/offline workings and experience, this are the relevant parts. Take into account that the game is not your usual online multiplayer experience. The key is that "No Man’s Sky at its heart is a single player experience, it’s just set in a multiplayer universe" (which for me it's actually a selling point, whereas for others expecting a "proper" multiplayer experience might be a drawback).

Anyway:

So all planets exist at the same time then? That must be an impossible to maintain server.
Not really, as it’s procedural, a planet or system only exists when you’re there, and the furthest things you can view are there only at the lowest level of detail. The stars you see are real stars, and the planets real planets, but they’re not taking up much space. If you can’t see anything at all, it’s not even generated.
Think of it like generating the world in Minecraft, as you walk, you generate more of the world – and things get pretty taxing after a while, because Minecraft is saving a lot of this information.
No Man’s Sky however throws this information in the trash when it’s not needed – the planet is simply regenerated from the same seed, if you will, when you return.
How does it handle your information? Anything you do is saved forever to your machine, that bird you killed, that terrain you destroyed, that blueprint you found, etc. but for everyone else, that planet is as untouched as the day you found it.

So how does the game handle two players being in the same area?
When you and another player are in the same area, unlikely as it is given the scale of a universe, with planets that can be the size of Earth or bigger, you’ll be placed in the same lobby – in this lobby data is shared. The terrain one player blows up is visible on the other players machine, and so on and so forth.

Sounds sweet, so my friend can see all the lewd things I’ve drawn in the sand? Nice.
Yes, but don’t expect much more than that. No Man’s Sky at its heart is a single player experience, it’s just set in a multiplayer universe. You can find your friends, but there’s no interaction to be had, no trading, no messaging system, etc. you can kill them if you’d like – but that wouldn’t accomplish much if you’re after a co-operative experience.
However, don’t give up hope – the developers at Hello Games have expressed interest in some basic multiplayer features, if enough people want it, and the game sells well enough to warrant continued development, we may see a co-operative multiplayer with a bit more depth. Another one of these wishes is ground vehicles, but currently, you’ll be scampering around on your own two legs.
You can be sure that if this is a hit, the developers will be continuing to support it, they’ve expressed this several times in interviews.

Edit: Formatting...

Do we have to be in gog galaxy to be online in NMS ? Me and a friend want the game but if we could bought only one game for two it would be good :D

From a support page:

I see, okay thanks :/ guess i'll have to buy 2 copy ^^

Depending on how the multiplayer aspect works, though, it might be the case that you can't be online with the multiple copies of the same license (different games use different systems, and I don't know how Galaxy-based multiplayer works in that case). Having it running in singleplayer on several dozen machines simultaneously should present no technical problem. But, yes, GOG asks you to please not share it outside your household.

The game installs on PS4 have no load screen or sign-in option, but are probably tied to unique disk or PSNetwork IDs. I assume for PC Steam they are tied to Steam Network IDs, and for PC GOG the installs themselves could generate unique player IDs upon first load, in order for online play to work. If this is the case, it's only logical for there to be a setting to choose a player name, as the game shows who found each location when it's attached to the server, and saying "This system was discovered by unique player 346e435-t63jh7gj3h5g7j4h-74jh56g746" would be awkward.

On a complete side note, but because it was included above...

"The same principle applies to movies - you're free to watch them anywhere you want, with anyone you want, as long as you don't share them with people who haven't purchased them. "

Uh, no, no it doesn't... people can loan out, trade, resell (garage sale), or gift movies and music... they just can't mass distribute them or display them for public viewing (theater), unless permission is given.

If I bought a physical copy of a Nintendo or PlayStation game, I could play it, and then give it to a friend for the weekend. I just couldn't make a copy and distribute that (at least not for-profit; free distribution may be allowed in some territories). This same principle applies to movies...

So, logically, if I wasn't gonna play NMS anymore, I could move the install files to disc and send them to a friend... I just wouldn't be able to keep a copy. But then digital works aren't physical works, and moving the files from one HDD to another, or any other media, is technically copying them. This complicates Copyright tremendously.

Sharing > Copyright ?

Society needs to evolve, sharing is how we got language and all the tech we have today in the first place. Why is it that modern tech and entertainment shouldn't be shared in a similar fashion; for the greater good?

Look at motherfucking Uber and Ballsack. This is what "sharing" is. Similar shit exists for gaming right now (I won't link to the naughty sites). Paid copying services are a race to the bottom in which only scammers win.

There's no other model possible for individual entrepreneurs under capitalism. The ransom model sucks. Libraries and grants are great in functioning democracies but aren't for everyone anyway and are completely unsuitable to shittier places. Basic income is awesome but, again, only works within the country. There are many ways in which copyright can be improved (slash term to 5 years since publication, abolish copyright on science, etc), but there's no working micromodel for selling creative works which doesn't involve an individual paying a small sum of money.