gasket prep is just thta, a prep. clean the surface real good, (duh), then brush on the prep. set the gasket on the prep on the pan, i usually put the bolts through backwards or use little clips to hold it in place, let it tack up for a few minutes, then lightly brush the top so it will seal to the motor.
i've only ever used it with paper or cork gaskets just for the extra help, but never with rubber. i dont know if you could or even should with rubber. you'd do best to read the bottle/can of the prep to know if you can.
there are brands for automotive use, same company that makes thread locker, which would probably work just fine, but when its a part that sucks to get to, or i really don't want it leaking, i use the aviation grade.