Is it normal for the iMac 5K's GPU to be at 90℃ with fan on constantly?
Powering that many pixels will be a stretch. You're powering a 5K display internally to your iMac and a 2560×1440 external display. Having your GPU at 90℃ is standard for powering that many pixels.

If you don't wish for the GPU to be that hot, disconnect the external display. It's still well within the thermal limits of the GPU, so I don't think it's something to worry about particularly. Apple always seem to run components hot for the sake of keeping the fans quiet, but you can override the fan speed to cool the GPU further using an app like iStat Menus.

There's a nice discussion about that GPU in the 5K iMac over at MacRumors forum:


AMD Radeon R9 M295X Core Clock Throttling, Heat, and Performance (page 34)