Ironically I genuinely believe he chose em cuz of the "goblin hearts" reasoning-
He also has that goblin language skill, I think he thought it would help him with that. But yeah, if you read the novel, Gongja deeply empathized with Goblins. Keep in mind his origins; he wasted 10 years as the lowest tier of hunter, and only finally awakened a skill because his heart was filled with envy.
Spoiler
Pretty sure it is because of the two times goblins can evolve
Can any others do the same once?
can someone spoil me, no shade to gongja but why tf did he choose goblins