Mesons are not considered to be gauge bosons of the strong force, because only the lighter mesons are thought to be responsible for the interaction responsible for the strong force.
The only remaining fundamental force that has no known gauge boson is gravity. The theoretical gauge boson for gravity is called a graviton.
All gauge bosons are bosons, which simply means that two or more of them can exist in the same place at the same time, unlike fermions. It also means that gauge bosons have a spin of 0 or 1.
Gauge bosons are thought to interact with the Higgs field. This theoretical field is believed by many scientists to be responsible for why some gauge bosons – like W and Z bosons – have mass, while others – such as photons – do not have mass.