"Firstmiddle Last" so "Caivsminvtivs Gallicvs"

And if 'Gallicvs' is a title, you have to use his birth name.

And you have to use the internal name of the trait group with the appropriate number. I'm not sure, but I think "Arrogans" is the Superbus traits (2 or 3), so something like:
give_trait "Caivsminvtivs Gallicvs" Superbus 2