Why gunbreaker?

[Iscah]

Japanese translation is basically "Gunbreaker"
ガンブレイカー --> ガン (gun) + ブレイ (buh-reh) + カー (kah) --> it's literally just gunbreaker in katakana

As to why "Gunbreaker", if I recall correctly Yoshi-P felt "Gunblade" was more the weapon they used and not the job itself, and he liked the idea of "Breaker" as it conveyed the sense of someone who was the vanguard that led the charge into battle and broke through the enemy lines.

This is dependent on my very limited knowledge of Japanese, but... laying it out like that reminded me that a manga author is called a "mangaka", so is that a general suffix for a person that does a thing?

If so, Yoshida's reason for choosing the name might be one element of it, but it's also something of a pun that resolves to an English word.

Or you can just put it down to Japanese people liking to use English words in things, but not necessarily understanding whether it sounds right to a native speaker. (I assume this is how we ended up with "tomestones", for example.)

Gunbreaker gunblades and Garlean gunblades are nothing alike.

That doesn't mean there's no connection between them. It's still likely that they were invented by one of those two civilisations, then the idea was taken and adapted by the other.