htop has GPU usage too, it's just not at the top by default.
I'm using 3.4.1, so it's possible you have an older version that doesn't have it.
btop in contrast, currently shows me: GPU utilization (graph/historic+current), clock speed, power-state, power usage, encoding/decoding utilization, VRAM frequency, VRAM bandwidth utilization (graph+current) + VRAM total/free usage and finally the current transmit/receive rate to/from VRAM.