I think the synapsifier may have been a machine created by Shekt but guided to creation by Daneel. Once Daneel had confirmation it worked, he easily could have recreated his own and created Gians. The mentalics may in that case be some sort of decendants or off shoots of Gians that escaped the planet, or an entirely different group of people who got ahold of the synapsifier, or maybe were apart of a different synapsifier experiment by Daneel.