MangoHud
Shows frame rate, temperatures and load on top of your games.
Installs from Ubuntu's own software archive.
Looking for MSI Afterburner on Linux? MangoHud does the same job for free, and TuxPick installs it for you.
Install MangoHud with TuxPickIt works on Ubuntu 22.04 and 24.04, and on systems built on them like Zorin OS and Pop!_OS.