If I wanted to, could I just copy my game files on a USB and give them to my friend? Surely you can't do that but from what I've seen it seems possible but I highly doubt it.

EDIT: I wasn't trying to say i was going to do that I was trying to. I was just wondering of there was anything stopping people from doing it!

GCHQ is on their way to your house right now, make sure you have clean underwear!

While there are no measures in place to prevent you from doing that, it is still piracy (if your friend hasn't bought the game), and GOG trusts you to not do that. Nothing stops you from doing that of course, other than your own moral fiber.

He could also just download the games, that way neither of you have to leave the house.

You can do it, but you're not allowed to do it.
I'd say it's not ethical or legal, so don't do it.
But if you're a total bastard, you can copy the games and give to all your friends, and even your rivals.
Mind me asking what you want to do with this information?

I could go into my tool shed, grab a hammer, and hit myself in the head with it.

What's your point?

Nothing.

If you do it then the moral responsibility is yours.
I just tend to share my GOG games with my brother.

I'd actually pay to see that, how much?

Technically, you can, and nothing stops you. You can play the game on any PC you wanted without hooks as long as it has the required operating system, by copying it into a flash drive, CD/DVD, the cloud, another hard disk, bluetooth, wifi direct, etc. As long as it can copy, you can do it.

Morally, it's up to you and your tastes. Personally, I think it is okay copying a game like that if your friend owns the exact same version. Say I have The Witcher 2 on GOG and my friend also has it on GOG, but he downloaded it first and my internet speed isn't up to par, then I could ask for his The Witcher 2 installer to be copied through a flash drive and save myself from the pain of downloading.

What, are my trousers undone? Oh I see your point. What, are my trousers undone...

From the legendary Bottom Live:
https://www.youtube.com/watch?v=6kNX_kmb4jo

While you have been answered several times already, of course I want to chime in as well.

There is nothing technically that prevents you from doing what you said (at least for the single-player part of the games, multiplayer part may have some restrictions...), but as you've realized yourself, it is still piracy. Some people have misunderstood earlier that because GOG is not trying to proactively prevent piracy (with DRM), it would be ok to give copies of the games to friends etc. NOT SO. Just because something is possible doesn't necessarily make it allowable.

So GOG and the publishers of the games are trusting you and not trying to prevent it technically, your job is to be trustworthy and not misuse their trust. You know you are going to hell if you do, hell has DRM all over.