The G3 is not a competition, and its not an award for best guitarist or anything. Joe Satriani holds the G3... its his concert and he invites people to come along, perform their music and at the end of the night jam on some hendrix or zappa or zz top ... i think its fuckin amazing and that would be my most anticipated concert other than an evening with Opeth... tons of great guitarists have gone on the G3 tour (i think there has been six) although a good number of them include Steve Vai, who was Satriani's student for the longest, so they are like a team...and although there is lots of shred at the G3 concert there is also tons of slow stuff... especially at the one with Satriani, Vai, and Eric Johnson...
as for the guys from Opeth at the G3... well, the G3 is a "jam" and we know that these guys don't jam too much... although I have noticed that they are starting to lean towards it a bit more... also the G3 is kind of a virtuoso thing, so I don't know if they really fit that sort of thing... Even Mike says he is no Jeff Loomis...
G3 is not an announcement that they are the best.. they just do it for fun, but a lot of people who watch it think that they are the best guitarists...
EDIT: BTW... if you've watched the G3 with Malmsteem you will see that Steve's band is just as front and center as he is...but I do admit that Malmsteem has a serious ego...