[The rest of the response presumes you're explicitly interested in working with Objective-C.]
Why not just draw with OpenGL into an NSOpenGLView?
I'm not sure I'd go with anything animated, not necessarily because it wouldn't work or be performant enough, but because -- why risk it?
Just use a single NSTimer to drive the frame redraw into NSOpenGLView. With SDL and most other frameworks where you control the main loop, you'd have an infinite loop which may or may not sleep upon frameswap, then you'd measure time passed. Under OS X, you'd have CADisplayLink. But if you're using NSOpenGLView under GNUstep, as far as I know your best bet is still using NSTimer.
NSTimeInterval delta_t = [self timeSinceLastFrame];
[self update: delta_t];
Some more thoughts: For a fullscreen game, there aren't many benefits of using AppKit under non-OS X platforms, on the contrary. Benefits are primarily with non-animated buttons and with 'generic-looking' text entry. But if you're doing windowed stuff, you're suddenly blessed with menus as well.
Your choice should definitely be only between SDL and AppKit. And this choice depends only on whether you're going to go fullscreen and whether you'd like to use pre-made UI widgets. For the game content itself, you should do your painting with OpenGL (even for text labels), at which point your use of Foundation should be the same no matter what framework controls your main loop.