It's so simple this hurts my brain.

Finally, a gunblade specialist!
Just because he's a specialist of a weapon doesn't mean he is the only one who can use the weapon in Garden. Essentially troops often learn how to use different weapons. In medieval Europe the average foot soldier or archer was trained in the use of Bows, Crossbows, Sword and Hammer or Axe. He specialised in utilizing his bow so that the enemy didn't get close enough and sure he couldn't stand up against a swordsman in close combat unless he was lucky because his training wouldn't lead that way. However he could use the weapon he was given for melee combat if he needed to as a last resort.

Just because Squall is a specialist in the Gunblade it doesn't mean he is the only one to have used or learnt the use of the gunblade in the entirity of SeeD he would have learnt the basics from the instructors same as Seifer Almasy would have. However Squall and Seifer would probably have been left to find their own style and techniques wether from visiting masters or perhaps in the same wa as say Jeet Kun Do martial Artists learn the basics but develop their own martial art style from there on. As seen in the intro and everything about the two Gunblade masters we prominently see in the game (and Laguna in the "movie dream" against the Ruby Dragon. Though his style seems very reminiscent of Seifer's) fighting styles are very, very different from weapon design through to moveset. I'm led to assume that they developed their own fighting style after a core moveset was learnt.