I agree, and have used GNB in most cases.
While there isn't a hard/fast rule for abbreviations, they tend to lean toward consonants over vowels. Looking at all the class/jobs across DoM, DoW, DoH and DoL, equalling 36, and their most common abbreviations. Not counting the five that start with a vowel (ACN, AST, ARC, ARM, ALC) There are a total of 24 that do not use vowels in the abbreviation, leaving seven that do, ROG, NIN, SAM, WAR, BLU, CUL, MIN.
It would stand to reason that while yes, there is a chance it would be GUN, it is much more likely it will be GNB.
