Sorry if this is the wrong place to ask but I was wondering, On [modded] there are tons of games that are free. For example these games can be 10-20 bucks on Steam but free on [modded]. Is it 100% because of donations?

It's 100% because of piracy.

What about cheaper games from GOG?

Deleted.

This explains everything. Thank you.

This one you are on is the legitimate official GOG site, the other one you are talking about is an illegal pirate site.

Thank you for the detailed reply. You seem to really know this stuff so I have 1 more question please. Sites like Kinguin that have very cheap codes for games on Steam, whats the explanation there? Thanks again.

The two most likely scenarios:
1) Game keys purchased with stolen CCs. High risk of getting the keys revoked later on.
2) Keys from bundles, that were bought in masses, and then the individual keys getting resold for a higher price again (but not as high as the original price).

Kinguin is a grey market key-reseller. this means that the keys you find there are not approved by the different stores, and the reason why they are cheaper is becuse they are often got via scams, stolen keys, keys mass bought on sales then resold and other illigitmate ways.

When using a grey-market seller, you need to be careful, beacuse if the keys become reported as stolen or gotten via scams, they might be revoked, meaning that the key no longer works and the game may be removed from your account.