I haven't been to the Cave of Tribulations yet. I've decided to do a second playthrough and go to it from there picking up any boss trophies I missed the first time.

I can give some tips based on what I've gleaned from various sources.

For one, consider defense. You can probably raise your defense to a point where Gabriel won't do damage to you. The only two characters to consider this strategy with are Fayt and Mirage, as they have the highest defense in the game. If you refine the Boots of Prowess enough you can synthesis the ATK/DEF +30% Factor to them. I'm not sure how exactly you gain more defense. Try refining Diamonds and putting them in your armor maybe. You can probably solo Gabriel with Fayt if his defense is high enough.

Keep in mind this advice is coming from someone who took 7 tries to defeat Luther, and then had to damn solo the second half of the final battle using Albel because my strategies were so inept.