Well.. The russian spelling of her name (which is in-universe canon spelling) is Гум ( Г=G у=U м=M ), and that's the japanese pronunciation also (also with distinct japanese accent), so it sounds like GUMU in the voicelines. Aand for some reason, the global/english translation of the game "changed" the reading of it to Gummy.
Do what you will with this information (maaaan, it's so satisfying that, for once, I can finally make use of these 3 languages that I learned(not bragging tho don't stone me))