Although there are compelling theoretical reasons to believe that gluons are massless, they can never be observed as free particles due to being confined within hadrons, and hence their presumed lack of rest mass cannot be confirmed by any feasible experiment.
[1][2] The graviton is a hypothetical tensor boson proposed to be the carrier of gravitational force in some quantum theories of gravity, but no such theory has been successfully incorporated into the Standard Model, so the Standard Model neither predicts any such particle nor requires it, and no gravitational quantum particle has been indicated by experiment.
The Weyl fermion discovered in 2015 is also expected to be massless,[3][4] but these are not actual particles.
The Weyl fermions discovered in 2015 are merely quasiparticles – composite motions found in the structure of molecular latices that have particle-like behavior, but are not themselves real particles.
Neutrinos were originally thought to be massless – and possibly Weyl fermions.