I have the following on Ubuntu 20.04 LTS:
Fan control: GitHub - nan0s7/nfancurve: A small and lightweight POSIX script for using a custom fan curve in Linux for those with an Nvidia GPU.
Temp monitoring: hardware sensors. “indicator-sensors” from snap.
As you can see over there, I haven’t been able to set it up right for my 3090 without breaking my multi monitor setup. But it works if you have only one or knows how to modify the xorg.conf file .