GNUStep is not a drop in replacement for OS X. Among other things, every framework isn't available. It's also only API compatible (or striving for) with OpenStep, which is, now, a subset of what OS X provides. It's not binary compatible, so at a minimum your OS X apps need a recompile. More likely, they need porting. They also went with a different look and feel than what OS X has.
Etoile attempted to provide something closer to the OS X experience, but there seems to have been very little movement recently, and it had its own ideas (some pretty interesting ones, actually).
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GNUStep is not a drop in replacement for OS X. Among other things, every framework isn't available. It's also only API compatible (or striving for) with OpenStep, which is, now, a subset of what OS X provides. It's not binary compatible, so at a minimum your OS X apps need a recompile. More likely, they need porting. They also went with a different look and feel than what OS X has.
Etoile attempted to provide something closer to the OS X experience, but there seems to have been very little movement recently, and it had its own ideas (some pretty interesting ones, actually).