I shall direct you to the crux of the matter, sir, it's no secret.
Starting at 4th level, a rage prophet’s spells grow more potent when he rages. When using moment of clarity, he adds his barbarian level to his caster level. At 7th level, he adds his Constitution bonus to the save DC of any spells cast while raging.
Combine this with Gestalt, and it's not hard to see how it falls together. Your Barbarian Level will be 18, for starters, and your Con score should be well past the thirties while Greater Raging.
The AoE save or Die I mentioned was just one possibility. You can cast 7th level spells, with a caster level in the upper 30s/low 40s depending on your focus, and a DC through the roof. Pick a nasty spell. Incidentally, you should also have over 300 HP with this build, so you're not exactly becoming a glass cannon to pull it off.
Happy optimizing, all!