You are overthinking this.
The Global Tracking Program is a seller protection.
Any tracking by any service is a seller protection.
As a buyer it is not particularly useful to you except emotionally.
If you are not home when the carrier arrives, you will get a Notice about where and when the parcel can be picked up.
If the parcel doesn't show up in your mailbox tomorrow, you can open an Item Not Received Dispute in the eBay Resolution Centre at the bottom of this page.
Give the GSP tracking number in the Dispute.
If it is real and the GSP did not deliver on time, you will be refunded.
If it is not real (much less likely) the seller will not be able to prove delivery and you will be refunded.
So GSP or no, tracking or no, if you don't get your purchase, YOU WILL BE REFUNDED.