I have a gasser 250 2006 & the exhaust rub's. All the gasgas bikes I have seen have the same problem, I havent done anything with mine as it hasent caused me any problems, in fact never give it any thought really, until now!!
Once washed plastic's always lose their shine, no matter how new or old your bike is, but once I've washed mine and blasted it off with a air line I use Autoglym Vinal & Rubber care, this always brings them up new till I go out again.