Quoting from the GSX manual, Page 26:
When the GSX Level 2 Expansion is Active, the Deboarding/Boarding settings in the Timings page will look different.
Since passengers are now no longer simulated, but will appear as actual animated characters, the original timings in seconds/passenger will not be meaningful anymore: the time required to board passengers will be a combination of their walking speed and their density. Each passenger has its own walking speed, which depends by its animation and walking style, but it’s possible to control the passenger Density, which will affect the number of animated passengers visible on screen at any given time.
Of course, if you are parked on the apron, the boarding speed is affected by the number of stairs, and if you are parked on a gate, the number of connected jetways affects the boarding speed too, just like in real life so, if you need to board a large airplane, you should likely park it on an heavy gate that have more than one jetway, and the airport you use doesn't have one, the ability to customize jetways is one of the main features in GSX L2.