Till now I have missed 2 GFs, Alexander and Pandemona, this won't be a problem, will it?
Well, as far as battle goes, no, Alexander isn't particularly necessary to beat the game, but he has some important abilities that will help you out later. For instance, he has the Med Level Up Ability that you need to aquire all the items to summon the GF Doomtrain. Also, he has the Revive ability, which comes in very handy in later battles. Finally, he has High Mag RF which lets you consolodate all those middle-level magic spells into their most powerful forms.

Pandemona, although he has some nice character stat-boosting abilites, doesn't have anything quite as important.