[enkiv2]

Not much would happen. Big chunks of what constitutes OS X is already open source. Nobody runs GNU/Darwin the same way nobody runs Plan 9.

Apple's market is primarily geared towards people who aren't interested in the tech and are paying for the logo. After all, anybody who wants an OS X equivalent system who knows what they're doing can buy a bog-standard PC and run some free unix on it with GNUStep. So, the licensing for Apple products doesn't matter because Apple's market excludes anybody who cares too much about licensing (or anything other than branding).

[Jtsummers]

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).