We can't run ANY banner on our GS6000 no matter what we do. Heat off, head high, nothing works. Epson installed the new media feeder for us claiming it would solve the problem, but it didn't. They finally threw their hands up and said "it's not approved media" and/or tried to blame the material itself (even though we can take if off the GS and throw it in our Mimaki and run a full roll with zero problems) and they couldn't do anything further.
Some people have no problems with heavy media at all in their GS, others (like us) cannot run anything thick in it at all. I don't know why it is, but there seems to be some serious issues/inconsistencies with the media feed system in it. We bought our GS specifically to print Korographics wallpaper media and after trying for a year, writing custom profiles, and having Epson technicians out several times, we've determined the machine cannot do it without getting head strikes.
Sorry to be the bearer of bad news but it sounds like you're in the same boat as us, you may not find a solution. I will say the cheaper the material the more likely it is to buckle from heat, Grimco's Pinnacle is pretty darn cheap so I'm not overly surprised it's warping. About the only thing you could do is trade up to a higher-end material...