Monocrystalline lattice matched $\mathrm{Er}(0.6\mathrm{at.}\%):{(\mathrm{Gd},\mathrm{Lu})}_2{\mathrm{O}}_3$ films with nearly atomically flat surfaces and thicknesses up to $3\mu \mathrm{m}$ have been grown on ${\mathrm{Y}}_2{\mathrm{O}}_3$ substrates using pulsed laser deposition. The emission cross sections were comparable with the ones of $\mathrm{Er}:{\mathrm{Y}}_2{\mathrm{O}}_3$ bulk crystals, showing only a marginal spectral broadening. Rib channel waveguiding could be demonstrated after structuring the films with reactive ion etching. Gain measurements have been performed and the results compared with a theoretical gain spectrum. A gain of $5.9\mathrm{dB}∕\mathrm{cm}$ could be measured at $1535.5\mathrm{nm}$ through in-band pumping at $1480\mathrm{nm}$ . The scattering losses in such a $7\mathrm{mm}$ long rib waveguide have been determined to be below $4.4\mathrm{dB}$ at $632.8\mathrm{nm}$ . An extrapolation to the gain wavelength with the ${\lambda }^{-4}$ -Rayleigh law of scattering resulted in losses of $0.2\mathrm{dB}∕\mathrm{cm}$ at $1.5\mu \mathrm{m}$ .