Put the pc that can do gpu's in a different venue work, school or home, instead of the default then. I do that all the time when I have different pc's doing cpu or gpu projects. I don't like doing both cpu and gpu workunits from the same project on the same pc, they workunits are different lengths and cause cache problems for me. So I put pc a in the default of no gpu units by yes to cpu workunits, and pc b in home and allow gpu units but no cpu workunits.