Fix bugs in which code? GNUstep? Third party GS apps? Your apps?
You should certainly fix them on the platform you intend to run the code on. Windows and *nix builds of GNUstep will branch differently for very obvious reasons, while on macOS you can force GNUstep into being built into your binary, it's just not something we suggest being done.
In your other email it sounds like you want to fix apps. First choice is "where will these apps be running"? If the answer is "freebsd", use freebsd. If it's "linux", use Linux. If it's Mac, use Mac along with Apple frameworks.
For non-Apple platforms, it's best to write GNUmakefiles that make use of gnustep-make helper scripts and forget Xcode is there. We have a tool to build Xcode projects, but it's not going to help you modify them. We have an IDE (ProjectCenter) which doesn't know about Xcode project files, and I personally find myself more productive if I don't use it. Other people like it.
Which applications do you want to fix? Do you have specific bugs you want to fix? Can we help you get started with a specific task?