•Presenting a new method to the off-design analysis of gas turbine engines.•In-depth understanding of the performance of the gas turbines and their components.•Introducing a comprehensive tool to perform the off-design calculations.•Off-design performance analysis of a micro-turbojet and a double-shaft gas turbine.•Use of a micro aero-derivative gas turbine as a driver of a compressor test rig. This paper presents a new method to formulate the problem of simulating and off-design performance analysis of gas turbine engines and organizing this problem to solve. This method, without any simplification, follows an identical procedure for off-design analysis of all configurations of gas turbines. The method is based on solving an equations system, modular and flexible, and requires the least user experience. Details on the formulation of characteristic and compatibility equations and subsequently formation and details on solving the equations system were presented. The performance of the Olympus-HP micro turbojet engine was obtained from this method validated against test rig and manufacturer results. As an application, the off-design performance of a double-shaft gas turbine was investigated, which is configured from the Olympus-HP core and power turbines that are corresponding to the loads. A centrifugal compressor and a fan with 45 kW consuming power with different design speeds were utilized as the load. The running line and power map of the double-shaft engine were shown. The effects of the environmental changes on the engine performance evaluated. This engine will be used as a driver of a test rig facility of centrifugal compressors of micro gas turbines (MGTs).