This script allows you to show the current GPU frequency of a Linux DRM GPU. It should work with both AMD and Intel GPUs, though Intel is currently untested.
Symlink or copy gpufreq.lua into your mpv scripts directory (e.g. ~/.config/mpv/scripts/.) Then
run ls /sys/class/drm/ to show a list of cards; usually your card should be card0.
Add a keybinding in your input.conf to the command script-message show-gpu-freq "cardnamehere"
where cardnamehere is the card identifier from the previous step. For example,
U script-message show-gpu-freq "card0" would show the frequency of card0 when you press shift+u.