Agreed. It's not necessarily "coincidence". In the case of "bang" appearing in separate, unrelated language families could due to the direct, iconic, onomatopoetic relationship of the sound to the word.Is it coincidental?
The Chinese bang (wooden club) is so called because of the sound it makes, which is also why the Scandinavian, Icelandic, Old Norse banga, also means to pound and hammer. Like on a drum.
"Bang...bang...bang". "Banga banga banga". A lot of words come from the sounds that we associate with them. "Choo choo".
Gun/Gwan means something like "to beat many times with a tree". Makes sense to me.
Many years ago (in the 1970s) when I took some college classes in linguistics, the role of onomatopoeia, rather than mere coincidence, as a source for such unrelated yet similar sounding words was still being debated. I have no idea if or how that discussion was resolved by academics.