Couple of things, its hard to compare the GNR reunion tour to U2 selling out stadiums on a tour, because one is a reunion of an iconic band that hasn't played with some of the original members in over 20 years. The other is a band that has never had a lineup change for decades and have been consistently releasing albums and touring. So I guess I'm not arguing that GNR are bigger than U2 (I wouldn't argue that) I guess I'm saying this reunion tour is a bigger deal than another U2 tour because the nature of it is completely different.
As to Garth Brooks, you are right, he is incredibly popular and him coming back was HUGE. However, whenever I was mentioning this being a big deal, I said it might be the biggest thing in rock music since the Zepp reunion. Country music is a whole different animal, country music is extremely popular in America, where as rock music is not so much any more. I'll put it this way, besides U2 I can't remember another rock band coming through my city (Pittsburgh) and selling out Heinz Field. There are several country acts that literally sell it out every summer. So I think its just different.
I guess I'm not saying GNR is the biggest band in the world or the most relevant band in the world (clearly they aren't), but this reunion might be the biggest thing in rock in while, that's all.