Don't give up!
I was just at Factory Card Party Outlet for my son's soon to be birthday, you should have good luck there! Even call ahead to ask if they have balloons, etc with Barney.
I am also Celiac so can relate to the cake issues. Just make sure if you get a cake transfer that its GF. You could even explain your situation to your local Krogers or Giant Eagle or Walmart or Sams Club, they may sell you a transfer or decorations since they don't sell GF.
My son loves Thomas and we found a single Thomas candle at Factory Card Party Outlet. I usually make cupcakes so this will be perfect! I have a cupcake "tree" and will put his on top, decorating the rest in basic decoration.
Good Luck! I am also a dietitian so if you need anything else GF, let me know!