Gamma rays are a form of radiation. Gamma rays bombard the inside of the Great Portal wormhole. When Hulk went through the portal aboard the S.H.I.E.L.D. Space Shuttle he was bombarded by the rays. Unfortunately, this meant that his strength was greatly lowered and it took quite a while to build it up again. This meant that when he crashed on Sakaar he was vulnerable to the natives and forced into the gladiator games.