amok: the,
yes, if they want to, they can do so.
Ancient-Red-Dragon: No, they cannot, unless they want to be sued in a court for really big bucks, and lose.
What you are saying they "can do" would be theft on GOG's part, which is quite illegal.
https://www.gog.com/forum/general/gogstaff_what_happened_to_southerns_account/post11 edit: to make it clear, when you buy a game on gOg (or Steam, or EGS, or Humble, ot Itch etc), you buy two products. You buy a game license, and you buy a license to use the stores services. the store cannot rewoke your game licenses (only the game license holders can do that), but they are allowed to stop the their services - which includes maintaining your library, access to the library, access to updates etc. the games themself, if you have downloaded them, are yours. so just makes sure you have your games downloaded if you want to talk thrash :)
edit 2: the same principles apply if gOg foldes, you are still legaly owning the game license, but you can off course no longer use gOg services. by the way, the email you get when you buy a game, is the proof of game license ownership, so do not delete them.
edit 3: i do not see a situation where gOg will actually delete and account which have got paied for games on it. they have many other milder sanctions before this last drastic step. however, they need to have this ability so thay can for example legaly delte spam accounts (what do you do if a spam account has a game they bought for $1 in a sale?) and they have legally the power to delete any account they want. doing so would be very bad PR, but not illegal.