N1, N2, Ng - different names, same thing. It's the gas generator RPM.
Large turbines almost exclusively use an air starter, as described. Smaller turbines can use electrical starters. Sometimes, this is achieved by running the generator 'in reverse'. A less common method are hydraulic starters, where a hydraulically driven engine does the initial cranking. Again, often this is a hydraulic pump going 'in reverse'.
On military engines which have to be startable without depending on external air sources etc, you might see cartridge starters or iso-propyl-nitrate starters, where the combustion gases from IPN in a chamber or a cartridge are sent through a turbine to crank the engine.
I thought I was doing good trying to avoid those airport hotels... and look at me now.