Geomungo is a Korean musical instrument. When I type this word, do I have to use capital G, or small g as in guitar? For the instrument is used in quite a small area and has not that popularity like the guitar, I'm hesitating to decide which is proper.
Whether you write it geomungo or with the older spelling komungo (which is prominent in Google Books search results), it is a common noun and there is no need to capitalize it unless you would do so anyway—for example at the beginning of a sentence. I took a look through the results linked above and through some Google main search results and found no reason to think otherwise.