The colossal squid is the world’s largest squid. Colossal and Gargantuan are synonyms. Colossal is similar to colossus, which is already taken by magic mountain. It’s possible that could be what happened here. I feel like Behemoth, Goliath, or Colossus work a lot better as a side name than Gargantuan. The word is just a little too awkward, especially with 4 syllables which I feel is just too much for an iconic name, Leviathan being the only exception I can think of. (All 3 of the other examples above are 3 syllables, El Toro has 3, Kingda Ka has 3, etc).
Honestly thinking more about it, leviathan and gargantuan are two very similar words. I think the reason leviathan works is because it’s an actual thing - a sea monster. Whereas gargantuan is just describing a size