Monolayer (ML) Ga2O3 with outstanding properties is promising for advanced nanodevice applications; however, its high exfoliation energy makes obtaining it challenging. In this study, we propose a more efficient solution to obtain ML Ga2O3 by exfoliation from indium-doped bulk β-Ga2O3. The exfoliation efficiency with the assistance of In-doping and the doping influence on the stability and structural and electronic properties of ML Ga2O3 are systematically studied using first-principles calculations. The exfoliation energy of ML Ga2O3 is found to be reduced by 28% and is of the same order of magnitude as that of typical van der Waals (vdWs) 2D materials. Besides, excellent stability is preserved for ML Ga2O3 at extremely high In doping concentration by phonon spectrum and ab initio molecular dynamics inspections. The bandgap of ML Ga2O3 decreases from 4.88 to 4.25 eV with increased In concentration, and the modification of the VBM converts ML Ga2O3 to a direct bandgap semiconductor. With the suppression of ZA mode phonon scattering, the pristine and In-doped ML Ga2O3 exhibit high electron mobility, whereas the strong electron-phonon coupling (EPC) effect significantly decreases the hole mobility. Finally, the transfer characteristics of 5 nm MOSFETs based on the pristine and In-doped ML Ga2O3 with varied In concentrations are simulated based on the non-equilibrium Green's function (NEGF) formalism. The Ion for HP has a maximum of 3060 μA μm-1 at In doping concentration of 5% and is triple that of the pristine ML Ga2O3 for LP at In doping concentration of 20%. The FOMs of n-type MOSFETs based on the In-doped ML Ga2O3 and typical 2D materials are compared and shows huge potential for sub-5 nm applications. Our study applies a new strategy for obtaining ML Ga2O3 and can also improve the device performance at the same time.