We don''t really know the details, but we can make an educated guess based off the lore on the Shadowbringers site and what we know about FF8.

What we do know is that the Gunbreaker job started off as a "Queen's Guard/Elite" position. Basically the elite guards/soldiers of the Queen of the Hrothgar. Not unlike how PLDs are the elite guards of the Sultanate in Ul'dah.

The Gunblade was likely developed as an answer to the occupation of Garlemald.

The skillset of Gunblade isn't really random at all. Their main abilities are all "fierce" sounding abilities. Gnashing Fang, Savage Claw, Wicked Talon, Ready to Rip/Tear/Gouge. The image is a very "beastly" one... which makes sense since it was developed by Hrothgar.

At the same time, the Hrothgar (or at least Helion Hrothgar) are stupidly loyal to the Queen. So the "Heart" skills make sense as well, and harken a bit back to the "Lionheart" of FF8. They have a deeply ingrained feeling of wanting to protect.

That's all I got for now, I might add more later.