Ever heard the phrase, "possession does not equal mastery"?
You're making an assumption that GoG has the game's source code. As I said before, this is not a reasonable assumption in most cases due to the number of times the rights have changed hands and the closure of the original developer. From what I see so far, they are able to make changes the hard way - that is, by performing disassembly and reverse engineering the program logic - but that requires a substantially greater time investment.
Even if the game depended on a third party service for some of the multiplayer features, they bought the entire license to the game, not just distribution. If you look at the publisher, it's GoG Ltd (Limited Company; subsidiary of CDProjekt Red).
Some of the game's online multiplayer functionality depended on both an online service and a piece of third party software needed to connect with it.The service has been defunct for fifteen years, and the software is useless without it. We have no reason to believe GoG has the license to modify that third party software.
As I wrote earlier, we can try to leverage the game's LAN multiplayer to play matches online through using software such as Tunngle to set up a virtual LAN. If you're up for testing if it'll work with Chaos Gate, by all means let me know.
It sounds like the setup has an "expired" IP set into one of the config files.
Not quite - the service has been defunct for over a decade. Just changing the IP to another one will do nothing, unless the server at that IP is running the same service the software client is supposed to connect with.