5'11" and I tend to buy 18/18.5" retro frames measure c2c not center top. The GF is 5'6" and I bought her a 16" c2c bike and that seems to fit her nicely, so I'd say you were after something around 17" c2c.
Kona's are sized weirdly (I think) so I haven't a clue about what there like, GT's come up on the bigger side apparently so take that into consideration.
GT wise depends how much you want to spend a Zaskar isn't silly money or a simple rigid early 90s GT seems to be doable for less than a few hundred quid. Happy hunting
1988 raleigh Cobra (always falling apart), 1993 Raleigh Mtrax Ti1000 (in bits),