Its commonly said that gauge bosons transform according to the adjoint representation of the corresponding gauge group. For example, for $SU(2)$ the gauge bosons live in the adjoint $3$ dimensional representation and the gluons in the $8$ dimensional adjoint of $SU(3)$.
Nevertheless, they transform according to
$$ A_μ→A′_μ=UA_μU^\dagger−ig(∂_μU)U^\dagger ,$$
which is not the transformation law for some object in the adjoint representation. For example the $W$ bosons transform according to