I have found that the GPU die on my iMac is around 90℃ and the fan is kicking in all the time. Is this normal? If not, how should I fix it?
I have iMac 27″ Retina 5K (late 2014) with AMD Radeon R9 M295X 4096 MB, and I have Dell UltraSharp U2715H 27-inch LED-Lit monitor connected to the iMac.
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:
This is probably NOT normal (I'm disagreeing here with the answer provided by user grg). I own the same iMac configuration you are asking about (late 2014 iMac 5k R9 M295x graphics card, 4GB vRAM). In addition to running the built in display at a higher 'scaled' resolution (2880x1620)—which Apple warns "may affect performance"—, I also have attached a 2560×1440 external display and a 3840x2160 external display. So it seems on that count I'm taxing my GPU harder than you are. But my GPU die temp (measured via iStat Menus) is generally around 75 degrees C, with my fan usually around a silent 1200 rpm (which seems to be its base rate).
My usual computer use involves web browsing, email, Microsoft Office, Slack, Zoom video conferencing, note-taking apps, etc. So not anything too intense from a GPU perspective (other than the fact that I have LOTS of windows open spread across 12 desktops in Mission Control, which does tax the vRAM). It will occasionally get above 90C when I'm doing something graphics intensive (such as zooming around a lot in Google Maps satellite view), but quickly drops down in temperature once that activity is over.
If you are doing something graphically intensive on a regular, ongoing basis (such as watching videos non-stop), that could cause your GPU die temperature to stay around 90C, but it would have to be pretty sustained for the temperature to remain there, as macOS seems to want to keep the GPU temperature below that point, and aggressively ramps up the fan speed when those temperatures are encountered, to try and get the GPU a bit cooler.
Also, if you are in an unusually hot room (and/or have sunlight directly on the iMAC or a heater located near it), that of course can affect the thermal situation.
I'd look to see if certain apps or webpages you might have open could be somehow 'secretly' driving GPU usage and temperatures. Try closing them one by one and seeing if that makes a difference. For instance, it could be that you have webpages open on sites with bitcoin miners, which will tax your GPU. There are browser plug-ins which will block miners, too. When doing this kind of troubleshooting, I find that looking at the "GPU package" power consumption (in Watts) using an app like iStat Menus (also mentioned by grg) is most useful, as it response most quickly to a change in GPU load, before that change is reflected in die temperature. For reference, my GPU package power use hovers around 40W–44W under typical usage (with spikes to around 60W when the GPU is being severely taxed).
It is not that abnormal for iMac GPUs to get hot easily, unfortunately. Make sure to clean the GPU heatpipe every few years (like 3 or so), even if you're not running in a dusty environment you'll be surprised how much dust accumulates there. You can find instructions on how to open your iMac e.g. on youtube or tell your dealer to open and clean it internally. If you are running 3D applications, think about limiting the frame rate and lowering the quality.
