I spent a couple dozen hours trying to tweak and tune my desktop this spring, get the watts down & see what Linux could do. CoreCtl was the best utility I found for the gpu, even though it left much to be desired & would sometimes work sometimes just not; my memory notably would get locked at max speed sometimes & not be adjustable, taking 20w right away. But it has the obvious knobs; adjusting clocks and mV per each state on each power profile, adjusting fan curves.

(Alas my motherboard appears to lack a ton of controls in Linux. I have been having to go into bios to do cpu undervolting and fan speed controls.)

More recently, I turned on the newer power tuning utility 'tuned' and it's been amazing. I'd fought down from 140 to 110w on my desktop but it still felt absurdly higher than it should be. Turned on tuned and now it idles at 85w. I haven't tried to sit and tweak it and see what it can do, but my impression is it's not as good at letting users tweak stuff endlessly. But it does do a ton of tweaking itself, and it's smart about adapting - switching to gaming profiles when games start.

I could be wrong but it seems like there aren't standards for motherboard management around platform details like fan speed control. And there's many many ways motherboards do things. Where-as gpu's apparently are just much more normative, are tweaked via pretty standard interfaces. Still, a lot is possible. I definitely recommend tuned, as a very all encompassing system tuner.