Useful documentation for running the GIMPS daemon can be found at:
http://www.mersenne.org/gettingstarted/
http://www.mersenneforum.org/forumdisplay.php?f=10

To run the GIMPS daemon as an unprivileged user some steps are required.

1. Before starting the GIMPS service run (as root) the following command:
  # /opt/gimps/mprime -m -w/var/lib/gimps
2. Answer 'Y' to the "Join Gimps?" question
3. Accept default values for the following question (or tweak them as you
deem necessary)
4. Stop the running tests with ^C
5. Exit the client, pressing '5'
6. Change the owner and group of all the files in /var/lib/gimps to
nobody:nobody:
  # chown nobody:nobody /var/lib/gimps/*

Then you can use `/etc/init.d/gimps start` to start a GIMPS client in the
background at boot. Have a look at /etc/conf.d/gimps and check some
configuration options.

If you don't want to use the init script to start gimps, remember to
pass it an additional command line parameter specifying where the data
files are to be stored, e.g.:
# /opt/gimps/mprime -w/var/lib/gimps"