Well, this is purely speculation (excepting that it was stated that it was for a movie), but Laguna was doing it for a movie--The Sorceress' Knight, I think. So, I bet that Seifer probably watched it when he was younger, and might have even based his style on it.

Alternatively, gunblade styles are gunblade styles, and he just picked the same one, but I like the first idea more. Also, Laguna mentions that gunblades were covered in basic training, but he didn't actually use them.