OSX 10.9 ships a 'seq'. This particular version was written in 2005 by the NetBSD project, and its provenance is NetBSD 3.0 -> FreeBSD 9.0 -> OSX.
Fwiw, it isn't a GNUism, though GNU's version was the first widely distributed one. A 'seq' appears in some old Research UNIX editions, and also in Plan9, but not in commercial AT&T Unix or in BSD. Instead 'jot' is the traditional BSD utility. Not sure if commercial AT&T Unix (and descendants) had anything similar.