Is it possible to limit the GPL to only the client side in a client-server product?

Question

open source

In parallel, I have already asked the question "Is it possible to use the code from the Unity Asset Store in an open source product." But I will duplicate the essence here: I started an open source project on Unity3D. This is a remake of the original game on a different engine, which is simply labeled "Copyleft".
Let's assume that the client part is written, the code from paid assets will not be used in the project, and I will go the way of inventing bicycles. Then I can license the game as GPL. The game client is distributed free of charge, and the source code is available to anyone who wants to get it.
But next, I want to add multiplayer with microtransactions to the game, which would help raise funds for support of the product. Does the GPL apply to the server application in this case, or is it not necessary at all?
I'd like to leave the server part as a "black box", making it harder to distribute someone else's copies configured to work with someone else's servers.
PS I'm not sorry, but it's not the same holy spirit to eat while working on an open source project.