Photons and gluons are the only two particles that we know to be massless and therefore capable of moving at the speed c (some neutrinos might be massless, but

Photons and gluons are the only two particles that we know to be massless and therefore capable of moving at the speed c (some neutrinos might be massless, but we know at least 2 of them are not). Since gluons are never observed as free particles and we're all familiar with photons, we call it the speed of light.