With a couple of small exceptions[1], the GNU behavior is also the historical POSIX behavior; the differences that will be jarring are where both BSD and GNU implement things that aren't in POSIX. Yes, the GNU behavior is GNUisms, but the BSD behavior is also BSDisms.

[1]: The most notable thing is that GNU generally allows flags to come before or after non-flag arguments, where POSIX requires them to come before.