It’s up to the scenery designers to implement, and a lot of them don’t add the roadways for service vehicles. In that case GSX tries to guess where they should drive from and it’s often quite far from the gate you are at.
The option is called ‘Speed up far vehicles’ in the GSX settings and helps reduce waiting times a lot. (I grabbed the screenshot from the manual, so might be slightly different on latest version):
The feature is described on page 31 of the manual. You can access the manual using the FSDT Installer app, via the ‘Manual’ button on the GSX Pro row.