I assume you meant to ask "how to build" and not "how to write", since writing the Objective-C code is trivial since it's all just AppKit and there's documentat

I assume you meant to ask "how to build" and not "how to write", since writing the Objective-C code is trivial since it's all just AppKit and there's documentation on Apple's website for that. Building for both GNUstep and macOS is difficult. The problem is, macOS wants you to use Xcode and GNUstep wants you to use their weird build system. In order to build software that is compatible with both GNUstep and macOS you will either need need to have two parallel build systems (one GNUmakefile and one Xcode project) or use a custom build system. Personally I think GNUstep's build system is stupid, and I don't like using Xcode, so I lean more towards a custom build system. Sounds insane I know but it really isn't that bad. Compiling and running a Cocoa application on macOS pretty much boils down to this: clang main.m -framework Cocoa ./a.out I don't remember how to do it for GNUstep. It's been a few years. Their GNUmakefile thing is REALLY weird, they have all these Makefile macros you need to import and they try to abstract all of this stuff away. Using Makefile macros. It's horrible. I asked ChatGPT and it generated this: include $(GNUSTEP_MAKEFILES)/common.make APP_NAME = YourAppName YourAppName_OBJC_FILES = main.m YourAppClass.m YourAppName_RESOURCE_FILES = Info-gnustep.plist include $(GNUSTEP_MAKEFILES)/application.make I would make a basic GNUmakefile, then run it with `V=1` (or `VERBOSE=1`? i forgot the name of the environment variable) to get the actual terminal commands its running, extract those, put those in your one-liner custom build script and be done with it