No gamma rays at these energies are able to penetrate the atmosphere and reach the ground. We only detect the particles indirectly. When a high energy particl

No gamma rays at these energies are able to penetrate the atmosphere and reach the ground. We only detect the particles indirectly. When a high energy particle enters the atmosphere, it creates a cascade of particles called an air shower. In that cascade, charged particles, mainly electrons and positrons, travel faster than the speed of light in air and thus produce Cherenkov radiation. This is light in the UV to blue visible range, you may know it from pictures of nuclear power plants. The telescope measure the Cherenkov light and from that reconstruct the properties of the particle that created the air shower. This event was not the first time we measured high energy photons on the ground, we do each night. But it was the first time we measured them from a GRB.