yeah you DO have a HDD though,right?
so do as Rockstar said(and i have done)
install the first disc to HDD and play the second disc(which actually first time you put the second disc in after installing disc 1 it also installs a little of disc 2)
I have noticed very little pop up and it runs fine
I've had many questions about 'The Amazing World of Gumball' gamerpic,it was part of a competition on the XBL UK dashboard a couple of years ago(download to enter kinda thing),and they don't seem to be available for normal download at all,and the competition(which i didn't win)has obviously ended