To reduce GPU temps, undervolting, IMO, isn't the right way to do it.
Idea of undervolting is to reduce GPU frequency. Thus, with lower frequency, less heat is also outputted. But not always the undervolt helps, especially when the GPU frequency is still stable when undervolted.
1st action towards reducing GPU temps is looking your PC airflow as a whole. Perhaps you have fans blowing into each other, maybe too much dust inside the PC. Fans aren't spinning as fast. Etc.
Once airflow is optimized but the issue still remains, 2nd action would be imposing limits. Here, rather than undervolting, i'd cap the FPS within the game. This gives far better results since rather letting GPU work as hard as it can, you instead cap the performance on certain level. E.g if you get 80 FPS in Cyberpunk 2077, you cap it to 60 FPS. <- Give it a go, look how things fare.