I do have the english version. I beated the boss and I am in the final room where I should use the gun, but I have no gun in my inventory to equip. Also, the quest where it says "take it back" (the gun) is not marked as completed.
Weird, dont know what happened then. Pretty sure I saw no chest I could grab or anything that signaled "take this for gun", I don't know what I did wrong then. Thanks anyway.