Hunters and FOs don't have it all that well then it comes to fighting Gal when starting out.
Fighting her effectively moreso relies on how well geared you are to fight it rather than player level. AFAIK, the best option for Hunters to damage Gal well is a Bombchu, which drops during the Triforce event, since you'll want a weapon that can hit multiple parts of her, and from a range. Next best option I can think of is maybe a Crush Cannon, a shot weapon that can also hit multiple parts that Hunters can use. Be sure to use anything that has high A. Beast attributes!
As far as armor goes, try using anything that gives good lightning resists, since her attacks chunk you based on your HP rather than whatever defense you have...
Which speaking of which, stack as much HP as you can. Once you're around the 1100-1200 range, you should be able to survive her most damaging move, her stomp, unless you're directly under her.
I know this'll sound weird, but DON'T use Jellen on Gal. The attack down makes her more dangerous, especially when she attempts to trample you when running around. You won't get knocked down when she hits you, which racks up multiple hits on you with one well placed foot on you, likely one-shotting you.
As far as the tornados go, best to just keep moving if you lack a way of really damaging her while she's airborne.
(This is very risky, but...) When she flies higher into the air shooting lighting bolts at you, if there isn't any tornados near you, you can shoot you multi hit weapon in the general direction she is in. (Use the minimap to find her) For whatever reason, she'll take massive damage since she'll be hit on all her parts... though I idk this only works for specific weapons that have infinite vertical range like Dark Meteor/Flow special.
On this server, monster in ultimate have been boosted, but there's a few that are especially rough to deal with if undergeared and doing solo. Gal, Olga Flow, and Sham, and Falz come to mind. If you're barely doing damage to Gal with your current weapons, an upgrade is definitely in order. Levels will definitely help though, but it could take a bit!