Gallade is largely outclassed by lucario. He isn't 4x resistant to stealth rock, he isn't 4x resistant to dark (justice heart), doesn't have a STAB priority attack, and he can't boost his special attack by two stages. Also, lucario has better speed, which is always useful in OU.
Either you havent used Gallade, or your just riding Lucario's nuts harder than P.Diddy on Biggie.
1. Gallade might not be 4x resistant to SR, but it still resist it, which is still a good merit..and for pokes like them it doesnt really matter about that. Both of them are weak defensively (except for gallade's special defense), so a few HP points wont matter on them
2. As much as Justice Heart is a merit (and it might just be me) but I rarely find an opportunity to use it, EXCEPT on Ttar which is already a big risk because unless I have full knowledge that they are choice banded and lucario NEEDS the SD then its not somethin i'm gonna consider over Gallade.
3. Priority, YES you are correct..Lucario does have better priority with either vacuum wave or e-speed..which is lucario's main selling point to use him over MANY other fighting types, not just gallade.
4. Why would Gallade ever need to boost his special attack? its really low, and he hits hard enough with his Attack power. Enough that after an SD it can OHKO/2HKO every wall in the metagame. The benifits lucario gets from his nasty plot is the ability to utilize vacuum wave to hit specific threats
5. This metagame is too fast for the both of them...I dont think I've ever said "It was thanks to lucario's 90 base speed that I won this game." As of right now, there is nothing specific for either of them to worry about speed and who's faster
Those are your reasonings to use lucario..now here is something lucario cant do:
Gallade has much more special defense, so it wont be OHKOed by so easily by any special attack, it can utilize a defense set with status moves such as WoW, Twave, and even though rarely used hypnosis. It isnt weak to the now popular mach punch, so conkeldurr doesnt revenge kill him easier than lucario...Works well with bulk up+Drain punch thanks to his special defense bulk..Can annoy physical walls with TRICK (underrated at this point)..and in general just a higher attack to hit hard right off the bat..
Anyways yeah, that does not sound like "completely outclassed" to me...im not saying either or are better..but they are both just different fighting types