Both of them are pretty good, but have their weaknesses.

Gan is a very powerful mage able to spam spells, but not as skilled as a melee fighter. Her only sword training were impromptu fights with their Esteem while she was having lucid nightmares in Ishgard.
So she tends to approach fights with brute force and does everything to overwhelm the opponent, hit them before they hit you sort of deal. Casts spells and slashes with a sword simultaneously, but that requires a lot of concentration. If Gan runs out of steam, their stance will be way more shaky and you can easily defeat them. And they need to rest up and eat a lot after fights like these.

After having to fight Zenos so much and undergoing Garlean training Alma is an excellent melee fighter but not as good as a mage, able only to dish out very basic spells and preferring to enchant her weapons or fists instead.
Her main tactic is to outsmart her opponent and use many makeshift traps, mechanisms and basic spells in-battle to catch the enemy off-guard. Dirty tricks like using culinary tools or kicking someone in the crotch are included, Alma is Not Polite
However, she is not that good at dishing out constant damage without any creative or underhanded tricks. As soon as her gizmos run out she's kind of useless and will tend to fall back until she comes up with a new strategy. That's when she's the most vulnerable