Timboli: This is no more than the game provider (developer or publisher) playing around with marketing and engaging in contract flexing. They are the ones who decide what is available to appear where.
So right now it is a laugh and half of ridiculous to see ZOOM Platform as any kind of threat to GOG. As for the first statement: no, it's not
solely the publisher's or dev's decision to push platform exclusives, rather, it is a
mutual decision which likewise involves the exclusive-hosting platform, who could always object to exclusivity conditions if they felt like doing the right thing for consumers.
As for ZP being a "threat to GOG," I have no idea where that notion is coming from, but it's not at all what the OP of this thread is suggesting.
The premise of the OP of this thread
is not that ZP in on the verge of causing GOG to go bankrupt or anything like that. Rather, the premise is that ZP is causing some games which should be & used to be available to buy on GOG,
not to be available to buy on GOG any more, because ZP has converted former GOG game(s) into ZP-exclusive games, when they ought not to have done so IMO.
So is that a "threat to GOG"?" No, it's not. But... Is that bad for GOG customers? Yes, it very much is.