A compact, single shaft gas turbine gas generator uses a high pressure ratio dual-entry single stage centrifugal compressor, can combustors, and a radial inflow turbine with the compressor configured to achieve an overall pressure ratio of about 12:1 or greater and Mach Numbers less than or equal to about 1.4. The radial inflow turbine is configured to provide an expansion ratio of about 4:1 to about 5:1, to provide partially expanded combustion gases to a free-power turbine or an expansion nozzle, in a work-producing engine configuration. A ball bearing assembly in front of the compressor is used in conjunction with a thrust piston assembly to take up turbine thrust load, while a radial tilt pad bearing assembly is used in front of the turbine. A collector with scroll-shaped sector portions collects compressed air from an annular vaned diffuser, and respective crossover ducts channel the collected diffused compressed air from each sector portion to a plenum feeding can combustors. Bleed systems at each compressor inlet provide increased diffuser stability margins. Gas turbine engines configured with the above gas generator may provide 30% thermal efficiency or more in a rated power range of about 4 Mw or less.