I've never used GC. But I have used metroplex for flukes while treating with chloroquine phosphate. Has to be the powder, I'm pretty sure a liquid like prazi has a bad reaction with the CP. I could be wrong on that.
Metroplex contains only metronidazole, not praziquantel... so it won’t actually treat flukes. It does treat other parasites, like brooklynella and uronema.
General cure has both metronidazole and praziquantel, the praziquantel part addresses flukes.
Prazipro has only praziquantel.
I generally try to avoid using praziquantel with other medications if possible, mostly because it severely reduces the available 02 in the water. But in a pinch, it can be done provided you are careful to add plenty or additional aeration with an extra powerhead aimed at the surface or airstone.