Germanium selenide (GeSe) is a unique two-dimensional (2D) material showing various polymorphs stable at ambient conditions. Recently, a new phase with a layered hexagonal lattice (γ-GeSe) was synthesized with ambient stability and extraordinary electronic conductivity, even higher than that of graphite, while its monolayer is semiconducting. In this work, using first-principles derived force constants and the Boltzmann transport theory, we explore the lattice thermal conductivity (κ) of monolayer γ-GeSe, together with a comparison with monolayer α-GeSe and β-GeSe. The κof the γ-phase is relatively low (5.50 W/mK), comparable with those of α- and β-phases. The acoustic branches in α-GeSe are well separated from the optical branches, limiting scattering channels in the phase space, while for β-GeSe and γ-GeSe, the acoustic branches are resonant with the low-frequency optical branches, facilitating more phonon-phonon scattering. For γ-GeSe, the cumulative κis isotropic and the phononic representative mean free path (rMFP) is the shortest (17.07 nm) among the three polymorphs, indicating that the κof the γ-phase is less likely to be affected by the size of the sample, while for α-GeSe, the cumulative κgrows slowly with the mean free path and the rMFP is longer (up to 20.56 and 35.94 nm along zigzag and armchair directions, respectively), showing a stronger size dependence of κ. Our work suggests that GeSe polymorphs with overall low thermal conductivity are promising contenders for thermoelectric and thermal management applications.