So you ask, why games on Itch have links to their Steam appearance, but you do not see games here that link to their GoG appearance?
I am not sure. My guess is, that you would need a finished game to realease it on GoG. The games you see on Itch that push their Steam are usually in the making. They try to get known, they try to self publish and try to get an audience. And steam does have early access. Though, GoG also has early access - but far less reach.
There are games that are on all three. Backpack Hero for example. They do have a link to the wishlist us on Steam. But I see no GoG link.