The impact of antisite disorder on the intrinsic Gilbert damping in well-ordered FePt films is investigated by time-resolved magneto-optical Kerr effect. The variation of mainly affects the electron scattering rate , while other leading parameters remain unchanged. The experimentally observed linear dependence of on indicates that spin relaxation is through electron interband transitions, as predicted by the spin-orbit coupling torque correlation model. Measurements at low temperature show that remains unchanged with temperature even for FePt with very high chemical order, indicating that electron-phonon scattering is negligible. Moreover, as decreases, the perpendicular magnetic anisotropy increases, and the Landau factor exhibits a negative shift due to an increase in orbital momentum anisotropy. Our results will facilitate the design and exploration of magnetic alloys with large magnetic anisotropy and desirable damping properties.