Maybe I'm a snob or maybe I just have a thing about pronunciation because I love language. Makes no difference to me, I'm still going to let myself be amused. And it's not a big deal. Again, I never advocated that anyone should change anything, I just found something odd and I stated as much. By all means, say Gwoemul if it pleases you.
Really, mispronunciation of Korean words doesn't bother me the way mispronunciation of Japanese words does. Japanese pronunciation is so straightforward, easy to learn, and simple for an English-speaker's mouth (therefore it's less excusable to hear repeat offenders)*, while Korean is... I swear, I was pretty certain my mouth couldn't make some of those sounds until my second semester learning the language. On top of that, the Romanization for Hangul is, as far as I can tell, super counterintuitive. I never try to pronounce a Korean word unless I can read it in Hangul first because the Romanizations are so f*cking loopy. Because of all of that, I never blame anyone unfamiliar with the language for mispronouncing anything in Korean but I still find it completely hilarious. Lord knows I butchered Gwoemul a number of times when I tried to say it before I took any classes.
For the record (and not that anyone cares), the proper Korean pronunciation of 괴물 (gwoemul) is something akin to "gweh-mool," where the "g" is pronounced a *little* like a "k," the "w" is soft and almost but not completely silent, and you say the "oo" without rounding your lips so much. Mind, even I don't bother with the subtleties when I'm just dropping it into an English sentence (like having two different ways of pronouncing Gamera) but I think it's fun to know.