(I'm using Tenrou Island characters, not the current ones, because that's when Hades was alive so it's more fair)
I don't remember much of Gildarts' feats back then, but Hades should win. If you look at his fight with Makarov, he threw Makarov in his TITAN form, he used a magical barrier instead of taking hits directly, and he casted an Amaterasu 100 that blew up 1/5 of Tenrou Island, so you can tell he was just toying with Laxus in comparison. And how close is Laxus from Gildarts' level? Because I'd say Hades with his full power must be at least 3 TIMES better than Laxus'!
More seriously: Crush magic has its limits against too powerful magic, otherwise Gildarts would be able to block even roars of Achnologia. Seeing how Hades has more range, he should start attacking first, and Gildarts would be forced into defensive against such devastating magic. While he might be able to BLOCK Hades' most offensive spells, I doubt he could completely PIERCE through them and reach Hades. Slicing Bluenote's black hole was impressive, but an Amaterasu 100 boosted by the Devil's Eye is something else entirely, and I'm pretty sure Hades could spam this spell with his near-infinite supply of magic.
So the fight would either last until Gildarts runs out of magic, either until he loses patience and runs desperately into Hades (wich means he'd get hit by Amaterasu and that would completely drain his magic), either until Hades finds the time to cast Grimoire Law.