While both machines are considered internal combustion engines that convert a fuel into mechanical energy, they differ in how they operate and the applications where they are used.
A gas turbine mixes compressed air with either natural gas or liquid fuels (Diesel or aviation fuel) then ignites it, producing high-speed exhaust gases that rotate turbine blades connected to a shaft that powers a generator or other machinery. They are used in large-scale power generation and aviation/marine propulsion systems.
A gas engine uses either spark or compression ignition on natural gas, biogas, propane, or liquid gasoline/Diesel fuel to push pistons inside cylinders. They can be used in electricity generation, industrial cogeneration, and transportation (buses, trucks) applications.