He has said it
twice:
First in 2009 during an LP of "Tom Clancy's HAWX":
https://www.youtube.com/watch?v=XQEAG_Y95Pw&t=301s (5:01 if it doesn't skip automatically)
and again in late 2020 during "Yakuza 7":
https://www.youtube.com/watch?v=AsazTShCEGk&t=144s (2:24 if it doesn't skip automatically).
Note the lack of doubt or hesitation when he says "Green Mountain Time" - this isn't a slip of the tongue he definitely believes this is what "GMT" stands for.
Edit: The Green Mountains are real and they are a mountain range that covers most of Vermont (directly north of where DSP grew up in Connecticut). I'm not sure which is stupider - thinking Vermont was so far east/west of Connecticut it would have a different time zone or thinking time zones change as you go north/south. He can't even blame TV/movies/Basketball American High School teachers for getting this wrong, the Green Mountains just aren't important enough to be mentioned in pop culture or history class.