(I'm assuming that was a typo for "Fishman" Pro EQ.)
If you run the output of the preamp into the GK's effects return, you'll be bypassing the GK's preamp, which probably includes its EQ---basically you'll just be using the GK as a power amp. If you plug into the instrument input, your signal will be running through both preamps.
Depending what you mean by "not enough power", the preamp may or may not help. If the limitation is your amp's power section, it won't get you any more oomph there, but if your instrument is putting out a weak signal that can't drive the amp hard enough, it should help.