It's expanded on in the lore book. Both Giaus and Regulus have studded older ways of using gunblades then most people use and both are very, very good at it. This is contrast to Nael who used a lance and Zenos who used Katanas. Giaus and Regulus are the only legatuses we've seen who use the traditional Garlean weapon, and they're both older then the other two legatuses we've met. The only other person we've seen use a two-handed gunblade is Nero I believe, but he's abandoned that in favor of his war-hammer Mjolnir. I'm interested if using more "common" weapons is a trend with the younger Garlean generations given that Garlemald isn't as isolated as it used to be.

Neither Gaius or Regulus are using normal weapons either. Heirsbane was given to Gaius by Midas nan Garland when Gaius was promoted to the rank of Legatus. Regulus' blade is a prototype he's testing for the Magiteck Academy to see how well it preforms in actual battle. I'd be very surprised if we ran into anyone else using a two-handed gunblade at this point. They don't seem to be considered a common weapon the way the one-handed gunblades are and even those aren't that common. The only people we see using them are the officers in the Imperial Legions almost all of whom are Garleans.