Hmm, that sounds a bit off... So I just checked my XFX RX 560. It's 29 degree's ambient in my apartment at the moment (so I'm a couple degree's off your temp), and I'm showing 44 degree's idle with the GPU's fans running at 0RPM (not spinning - visually checked to be 100% sure). After 1/2 hour of GTA V, I show 63C with the graphics cards fans sitting at 1200 RPM (I'm looking at Radeon Wattman for this info). Now I wouldn't expect your system to run as cool as mine since my case is large(ish) and I know it flow's air fairly well with three 120MM at roughly 600RPM fans blowing out (one in the rear, two in the top), and two 140mm intake fans (front lower bottom) at roughly 850RPM (according to speedfan). Have you tried running case fully open (no covers on) - if you have an air flow problem you should see lower temps...
Oh crap....I miss read your graphics card, so these differences are going to be way off (a GTX 560 is everything and anything but an RX 560)....