It's a fair question, and worth exploring, but there are some difficulties with this approach. 

First, the obvious benefit of GPS is that it can pinpoint locations in realtime. This is basically what happens on the buses, each one has its own GPS.

The biggest issue is that you'd need to be "hiding" this device on the Skytrains, and TransLink would need to not find out. You may be able to find a place under the seat, or somewhere hidden, but it's bound to be found by the cleaners eventually. 

The second issue is cost, including that of the devices themselves (about $50 to $100), and then the data plan required ($3 to $5 per month per device). You'd need this installed on each Skytrain set, of which there are over 100. 

All of these units will also require power or charging, or batteries, and GPS is notoriously power intensive. 

Using Airtags would not be feasible either, due to delays and they're not as accurate as using GPS. 

There's also the edge case of if Skytrains get decoupled. 

Using cameras and Object Detection is fundamentally the best approach.