There are many ways to get good magic spells. Check out Item/Magic FAQ (part III) by Cidolfas at GameFAQs for a good list.

I haven't tried beating Ultimecia with Squall only, but I don't think it will be very hard if you're junctioned right, which means you should definitely get some good magic to equip him with if you're going to attempt it.

You will not need any GFs for their damage if you have a decent strength stat and some Aura spells (which make it possible to get Limit Breaks even at high health).