I've only played one Gish character at the levels mentioned, my Core Rulebook Warpriest of Cayden Cailean. The Warpriest meets all the spellcasting requirements easily. So the question became, what about the 60% of a Barbarian damage? So, I ran some numbers... but only after I converted Ellisar to Player Core rules, because quite simply, they're better.
First, let's set our baseline: I went with a Giant Instinct Barbarian with a Greatsword just to keep things simple. Expected Damage output per round assuming two Strikes per round: Between 39 & 45 pts. I gave the Barbarian Brutal Critical, but I'm not great with Barbarian builds... I might be missing something.
My Rapier-wielding Warpriest would be off 60% of this pace by 3-4 pts per Round. Still, closer than might be expected. Key Feats: Sanctify Armament, Lasting Armament, Eternal Blessing.
A Ragathiel Warpriest, using a Bastard Sword in two hands for d12s, would meet or exceed the target quite easily, by anywhere from 2 to 18 points depending on level.
(Assuming attacks on an at-level Target with high AC.)