I've done a bit of research, I know that GOG is DRM free but I just want to double check quickly to make sure I understand the following.

A) Games download as a .EXE file so that at a later date if I were to format my PC, I could just run the .EXE from my backup USB HDD and that's it?

B) GOG Galaxy is optional but mandatory for updating your games right? Otherwise how would one get the updates.

C) When a game updates, say Divinity Original Sin 2, does it download just the update that I can backup in case I need to re-install game at a later date? or does it have you re-download the entire game again at the latest version?

I'll bite.

A) If you download the installers rather than using galaxy, yes, you can save them indefinitely and reinstall them so long as your OS still supports them.

B) No, Not really. GOG apparently has opened up Galaxy for devs to directly push updates, so they might get there faster, but theoretically the updates still come to be directly downloaded.
I say theoretically because sometimes things happen and updates don't seem to appear. If you do some searching around the forums I'm sure you'll find a few threads. None of the games I personally own has been hit by this, so I can't can't tell you any specific games, nor can I say if said missing updates were available in Galaxy but not on the website. I know there are some games that haven't received any updates, through Galaxy or otherwise, unfortunately.

C) That depends. Some of the games never seem to get update patches and only get all new installers. Some of the games seem to get installers that just.. break.
Pillars of Eternity, for example, broke for me every time I tried to use the patches, and I ended up having to completely uninstall/reinstall for the updates.
Another example I can think of is Terraria. I seem to recall every update being a whole game installer, but in that case the installer was only a few hundred MB.
In the case of Terraria I was able to install over the old installation without any trouble though.

Yes. No.
GOG routinely updates offline installers, and sometimes also provides upgrade patches.
At best situation, offline installers are only delayed hours to days, just after GOG Galaxy's latest updates.
However, at worst case, offline installers could delayed for months. If your updates come from GOG Galaxy, sadly it can not be backup for now.
As I have said, GOG do provide offline upgrade patches sometimes,
BUT GOG often omit them recently, only provides latest new version of full installers.

--
I have over 1000 games here and I check updates for GOG offline installers everyday.

Thanks to everyone who replied, much appreciated. So I should download the game directly from GOG.com then if i want it as an installer to backup. Offline patches is awesome, as I can back that up, Otherwise anytime a new update comes, I can re-download game and back it up. Sorry I'm very OCD with this stuff and GOG seems like something I'm going to enjoy dearly, not that Steam has ever been bad to me, but this GOG stuff looks neat, I just found out about it not long ago. It can be scary sometimes to jump over to something new.

Yes. The installers (especially the classic ones, without Galaxy) are DRM-free and unrelated to GOG website (well, aside from the advertisement you get during the installation process). Moreover, they can be extracted with tools like innoextract. Some updates are released as patches: in that case you can install the patch over the already installed game. But some other times, mainly when the game receives a major update, the whole installer is updated, meaning you have to redownload and reinstall the game you want to update. See answer above: you can backup either the patches (provided as small installers) or the updated version of the whole game installer.


EDIT: I see others have already answered your questions, and probably more clearly. :-)

And if you use gogrepo.py everything will be done automatically.

You have some great answers above, let me just add:
You can use galaxy and by setting the option download a backup copy as well.
Some games are not like this (to varying degrees), gwent is fully online only, others are largely online = witcher adventure game, absolver, goblins inc, tooth and tail, for example.
Mostly you won't see patches nowadays, all I ever see is a full installer release and these can be large. I would choose what you want to keep (for instance if a specific version works with a mods you may way to keep it)
If your interested in any multiplayer a large proportion require galaxy.
If you want online save backups then you need galaxy (or do it manually like any sensible person).
You will still see galaxy components and some have galaxy/gog folder structures created (dragons dogma recently), you cannot get around this, it should do anything with galaxy installed, but make sure your firewall blocks everything by default, grim dawn for instance likes phoning home for news for instance.
Finally be careful what you download, classic - ie installer not bundeled with full galaxy package - may be hidden behind the other menu, check there first and check the small print below the download area on the game page in your library. They are trying every trick in the book to get galaxy on your machine.

Thanks for the info. Much appreciated.

Welcome to GOG!

Here you can change your avatar: https://www.gog.com/account/settings/personal

What's wrong with the avatar?

Just to throw out into the conversation, the old GoG Downloader does tell me what games have been updated.

https://www.gog.com/downloader

I haven't tested it to see if it's correct but seems to work fine. I believe it makes you redownload the entire game though.

You're one of the sheep.

Become an individual like the rest of us.

BBAAAA BBBAAAA BAAAAA :)

(Some of us work visually and associate with the avatars instead of looking at names. Not sure if that's the reason why it was pointed out but I wanted to mention it.)

Nothing really but this person might, or then not, want to change it. I was just trying to be helpful.

I picture it more as the guy fawkes v for vendetta, anonymous, edgy hero fighting the good fight. Or I am just too lazy to change it...much like the 488 which was a bug from ages ago.
Just kidding as I have the same :o)

I stuck to the default for a long time myself, though a few more weeks than I wanted to as it took that long for the change to take hold.