Pepi, make a file named for example seti_gpu.sh and inside put:
#!/bin/sh
while true
do
for arg in `ps -Lo lwp --no-headers -p $(pgrep setiathome-6)`
do
renice 0 -p ${arg}
done
sleep 5
done
and then run it from a console (it might have to be a root console in your distro or run it with sudo).
You'll need to have the bsdutils and procps packages installed.