If it's free, then you are the product. That's an adage that holds true in most cases, and in this case, your code is being used to train Copilot, among other things. Even if that weren't the case though, and the service really was as benevolent as you seem to believe it is, would you be sitting there clapping as the service gets worse for a decent subset of users? Is that exciting and interesting to you?
Even if your open source code is on GitLab they can still be used to train Copilot. Most permissive license allows you to train AI models using them. Any license that forbids AI training would not be considered "open source" under most definitions.
Being on GitHub just makes it easier for them as they don't need to manually clone it from a different service.
