Gas Turbines have starter motors - they are in the bullet shaped protrusions right at the front of the Intake and are refereed to as "Starter Bullets" - they can be driven by air (from the APU) , hydraulics and, of course, electricity.
Air Turbines are generally used on large Gas Turbines and Electricity is geneerally used on smaller Gas Turbines
FWIW - APUs, many of which are Gas Turbines, also have a starter motors.