Okay, so G.M. stands for Gundam Mass-production. So, ironically, despite how people want to strictly classify Gundams-- those things are technically a type of Gundam.
But, putting that little twist aside, there is something one has to understand about the Japanese language and culture. Even when they want to put something in English to make it “cooler”, they butcher the English language to suit their own needs. It is kind of weird, but English is used liberally in fashion, games, and anime, to the point that it isn’t unusual to see children with something really inappropriate splashed across their clothing in big block letters and plenty of Japanese games are easy to play even if one can’t read any hiragana, katakana or kanji because everything vital to know is already in English…
In fact, any Japanese person who has graduated high school has taken 6 years of English language courses. And plenty take supplemental courses above that at cram schools. But less than one in a thousand is capable of halfway decently holding a basic conversation in the most simple of English. Oddly though, Japanese use English words every single day-- but turn it into Japanese letters leading to the word being absolutely butchered beyond recognition. They use the wrong vowels, add extra vowels, replace some of the consonant sounds… this happens to some words English has borrowed from Latin languages, but not nearly to the same extent.
Furthermore, Japanese LOVE to reduce words down to short two-syllable words. This is particularly true if the original name of something was two words. The most famous example is turning Pocket Monsters into Pokemon. This is incredibly common and done with quite a lot of things.
When one understands this, one can begin to look at what it would be natural for a Japanese person to do to G.M. So the first part is not too difficult. Ji sounds like G. So that part is not difficult. It is the M that gives problems. The most correct way of doing this would be Em(u). If one used this, and paused in the middle of the word, then one would have Ji-Em(u) which would sound very close to how it is said in English.
But, again, Japanese-- even when using English words-- could care less what sounds right. In Japanese one rarely switches from one vowel to another in the middle of a single word and certainly doesn’t put a stop between two vowels (although it is very common for words to have a stop between two identical consonants). As such, they aren’t going to say Ji-Em(u). They are going to drop the E sounds from the word and end up with Ji-M(u) or, when said quickly, Jim(u).
So armed with this understanding you have a choice…
- The series is created in Japan by Japanese. Therefore everything the Japanese do is right within this context. Therefore the correct word to use is Jim(u).
- This is an example of Japanese attempting to use English and failing utterly, thus butchering the word they are using. As a native English speaker, one can naturally and easily use the word they are attempting but failing to use. So, use Ji-Em(u), because that’s what they would be saying if it was proper and natural for them to do so.
I often face this problem every day. When I see an English word being used, I either have to try to speak it in the Japanese way screwing with certain consonants, using the wrong vowels and adding unnecessary proper vowels in order to make sure I am understood, which makes me feel a bit embarrassed as it sounds absolutely stupid to my ears and probably to others as I struggle to say the word incorrectly… or I can say the word properly in English and have a 50/50 chance of getting a blank stare in return as they have no idea what I am talking about.