>>8709562
In the beginning you won't have the ability to junction magic to every stat so you'll have to decide which GFs go best with which character. The one that really matters is giving squall a GF that has STR-J because the gunblade > anything else. As for the rest, try to evenly distribute the junction abilities, giving someone redundant GF abilities is pointless.
Which spells you junction to which stats really depends on what you have, but it's pretty intuitive. Attack magic works best for strength and magic power, life and recovery magic works best for HP and Spirit, etc. You can see the results for yourself so just try experimenting and see what produces the biggest stat boost
And something obvious but very important: make sure your GFs are learning the most helpful abilities, if you don't manually set which trick to learn they learn the shittiest ones by default. You want them learning stat junction abilities and refinement abilities, their HP and summon magic power are pointless unless you're spamming summons, which is completely unnecessary. Pay particular attention to card mod since it'll get you cool stuff for refinement and Diablos' encounter-half and encounter-none abilities since those let you switch encounters on and off (use the lamp as soon as you get it and beat him with limit breaks but remember to save first in case it goes wrong). Play cards, refine cards to items, refine items into magic, and there you go