In my earlier post I was not saying that it was impossible to cast magic without the aid of a GF, there could be other ways like the JM, I was saying that it is not an inate ability of the people in the FF 8 world. If it were possible to cast magic without a GF or some other aid then the main characters would be able to.

Also the inate ability of monsters to cast magic does not really apply since technically GFs are also monsters and so you could assume that all monsters are just magically inclined and so they are able to use magic. Or if you dont like that you could even assume that monsters could be able to junction GFs considering that Selphie even says that she found a GF inside a monster.