I would take it with the GM personaly, insted of in /g chat, just explain to him that you want to progress further, and you do believe there is people in the current raid team that is holding you back. No need to name and shame or anything.
Then after clearing it with him, do a friendly "I have been accepted to a new guild, wich I think will suit my skill and desire to progress, more than this guild, I wish you all best luck in further raiding and see you around!", as you can always raid with and alt or something with them if its needed, and its fits your scheduel. Thats atleast how I did it, though I never talked to anyone of them again.
See it as quitting a job because you got accepted to a new job with better pay and/or workhours.