Foundation - GNUStep vs iOS

I'm now up to a point in my project (that will never see the light of day!) where I need to write some code for the model part. As such, I don't need UIKit to write this code, since it's simply going to be in-memory data structures and methods to manipulate it depending on the view controller's interpretation of the input.

Since I don't need UIKit, I thought I'd install GNUStep on my Linux server, which I can access from any computer and write the model code using their implementation of Foundation.

First Class I try to write, I end up finding that NSXMLParser is declared as an informal protocol on GNUStep vs as a formal protocol on iOS. Now I'm left wondering, are the Foundation frameworks that different on these platforms that I'm basically wasting my time trying to set up GNUStep to write Foundation code for iOS ?

Any of you have experience doing this ?