You can only purchase a GMPP Major Guard plan while the vehicle is under its original bumper-to-bumper factory warranty (3 years / 36K miles).
There is a tiered pricing structure for this type of plan.
0 months to 12 months / 0 to 12000 miles (cheapest overall rates - GMPP new)
12 months to 36 months / 0 to 24000 miles (second cheapest overall rates - GMPP used)
12 months to 36 months / 24000 to 36000 miles (most expensive rates - GMPP used)
I posted some additional info in
this other post.
I've purchased two GMPP Major Guard plans from Dennis Fichtner. He's usually offered the lowest price for a GMPP Major Guard plan.