You can't undervolt the GPU through rmclock program.
This program just manages the states of the processor(the CPU).
If you preffer, you can use NCH :
http://www.pbus-167.com/
It is very easy and I think there is nothing which can hurt your computer.
Because I used rmclock just a few times, I don't remember if there there was an option to undervolt, but I think it's also safe.