Gargantaur might still be weaker. Really sucks to have two bottom tier mythics in the same kingdom, though, particularly where they are outclassed by so many of the lower rarity options in the same kingdom, which are in and of themselves not even that good.
Relevant idea on how to buff him:
Doubt anything will change this close to release, though. And after release, even if it does get buffed, we are more than likely going to see a pure numbers buff than any significant redesign, unfortunately.