Gamma Radiation was being tested by Bruce Banner when he was exposed turning him into the Hulk.
History
The Incredible Hulk
Bruce Banner experiments with gamma radiation and then turns into the Hulk.
Thor
Erik Selvig mentions a scientist who experimented with gamma radiation.
The Avengers
The Tesseract is mentioned to emit traces of gamma radiation, leading to the recruitment of Bruce Banner to assist in finding it.
Avengers: Endgame
Through further gamma radiation experimentation, Bruce Banner manages to figure out how to merge his intelligence with the Hulk's physical body. Later, Banner states that the radiation emitted by the Infinity Stones is mostly gamma so he can survive it unlike the rest of the Avengers. Though Banner successfully uses the Infinity Stones, it leaves him severely injured and later kills Tony Stark when he uses the Stones to destroy Thanos and his army.