You don't need either one. GPO comes with a Kontakt Player which hosts the samples for you. It is very functional. The only reason you would need full Kontakt is if you want to edit the sample data. I'm not sure, though, if it will load into Kompakt, but I don't see why not. Unless GPO's encryption prevents if from being loaded into Kompakt.
It may load in Kompakt. If it does though, you will lose many features that are programmed in Kontact such as the legato feature. The Kontakt player is actually more powerful under the hood than Komakt.