Ok, just started on PrimeGrid. Looks like both CPUs and GPUs are working. Not sure if it's utilizing all 9 GPUs though or just one per computer, and I might've screwed things up for BONIC by BIOS flashing these GPUs, will have to wait and see maybe to compare their workrate. Looks like 1 out of 3 is recognized properly as an AMD MI25.
So if you want to stop it from running CPU you should uncheck CPU here:
And then lower down in the project selection pick the appropriate flag for the GPU portion you want only here:
Then you just need to make sure your boinc client has both PrimeGrid and either Universe or Numberfields added and they are setup in Their specific preferences to use only CPU.
The boinc client should try to get work from each of the project you have added that are active in the client based on the project preferences so it will grab CPU tasks for Universe / Numberfields and GPU tasks for Primegrid