GSX includes a pushback truck for smaller planes, which will appear if your airplane total weight is less than 175.000 lbs.
The Pushback you have in the screenshot will appear ONLY if the airplane total weight is MORE than 170.000 lbs., which doesn't seem normal for a Learjet so, either you have an outdated GSX, or that simply shouldn't happen, unless the plane really weights that much.
I checked the version history, and that Pushback model had that weight limitation since GSX has been released.
