Hallo!
I killed Deathgaze in the Soul Shrine before I found him on the world map, meaning I can never get bahumut...
How do I get flare now? I have the GBA version...
my Celes has flare, somehow.

sure you can get flare without bahamut.

bad news is... it only works with Terra and Celes.

They can learn certain spells when they reach certain levels.

Your best bet would be to verify at which levels Terra and Celes learns the magics.
For Celes, flare is learned at level 81.
For Terra, flare doesn't appear in her magic spell list... :O

I verified my information at this website:
http://finalfantasy.wikia.com

You can check it out too, if you want.