From the aspect of story, is Gargantua a harmful or evil monster? Its description text isn't implying that at all.
I tend to avoid attacking monsters which are innocent.
It's nothing too serious. I just want to know if Gargantua is an evil monster, from the in-game world's perspective.
I follow my own moral standards when playing the game. Therefore I usually do not attack monsters which are not evil.
There is not much info about monsters but the game describes them as "Explore exotic magical lands thrown into chaos by terrifying monsters and powerful enemies." Add the fact that they try to kill your entire team and I'd say they are all evil. No?