They mention in the faq that they do not use GNUStep's implementation of the frameworks, but they do use GNUStep's implementation of the libobjc2 runtime. There's licensing and goal issues. Apparently their Cocoa implementation comes from "the same source" as Darling's, but I don't know what that means. (Cross referencing with Darling's faq, I guess it's The Cocotron.)