In this paper, an efficient ${\mathrm{Ge}}_{25}{\mathrm{Sb}}_{10}{\mathrm{S}}_{65}$ (GeSbS)-loaded erbium-doped lithium niobate waveguide amplifier is demonstrated. By dimensional optimization of the waveguide, an internal net gain of approximately 28 dB and a maximum on-chip output power of 8.2 dBm are demonstrated upon 1480 nm bidirectional pumping. Due to the improved optical mode field distribution within the active erbium-doped lithium niobate film and the mode overlap ratio between the pump and signal sources, a 15% high conversion efficiency can be achieved at a modest pump power of 45 mW. Furthermore, the noise figure of the amplifier can be maintained below 6 dB for low-input-signal power levels. Compared to state-of-the-art erbium-doped waveguide amplifiers (EDWAs), this heterogeneously integrated device shows superior gain performance at the desired optical C-band while avoiding the complex plasma etching process of lithium niobate, providing an inspirative solution for power compensation in the optical telecommunications.