Just watched a gear oil change video on a GTS Vespa.
The drain plug is on the back side, next to the tire - and is reached through the rear wheel spokes. Then you drain oil onto the tire, unless you rig something up.
You must drop the engine to do certain other common servicing.
S'why I bought a genuine Piaggio service download ($8) before siging for my Liberty 150i. Since I do all my servicing - I won't own any scooter unless it's going to be easy to service.
Too many owners of difficult to service scooters - just ignore their service schedule.
Stig