A low-energy beta electron
The energy carried away by a tritium electron is exceptionally low as can be seen from this comparison of average energies from a variety of beta decays: 5.7 keV when compared to several hundred keV for the others. The total energy liberated, shared between the electron and the antineutrino, is 18 keV. As the decay directly produces a ground state helium nucleus, there is no excited state and hence no gamma emission.