I've been wondering the following: How does GOG gain distribution rights to a game when its developer and publisher no longer exist? Let's take Slipstream 5000 for example. The Software Refinery (the developer) and Gremlin Interactive (the publisher) have long since disappeared, but GOG still managed to get Slipstream on the catalog. Who owns the copyright? Where does the money made from the downloads go? (Apologies for my bad english and possible obliviousness.)

While the former publisher/developer may no longer exist, the rights are still in possession of someone, usually passed on to company that bought / assimilated former holders of said rights.

Yeah, if you have a look at the game page of Slipstream you will notice that it says "The Software Refinery / Funbox Media" From that its safe to assume that Funbox own the right to Gremlin's old stuff and are the ones making the money from it
They have a few games here
Funbox page

For example, publisher you mention was absorbed by Infogrames, which in turn became Atari, while the rights to the game distribution were sold off to Funbox Media, who cut a deal with GOG to sell the game.