I got the 5yr/100k deal but I put a lot of miles on my vehicles so I didn't even pay attention to the year mark - I 'THINK' its from the date of purchase IE the day the warranty began to cover the vehicle but don't quote me on that one.
GMPP's start from when you purchase them.
For example, if you have a 2 year old car with 20,000 miles on it, and buy a 4 year 60,000 mile plan, your warranty would expire 6 years from purchase or when your car hits 80,000 miles on it.
Keep in mind though that Ally, which runs GMPP, is not stupid about GM vehicles. They know that problems occur after 7 years and 100,000 miles.
If you buy a GMPP when your car is new, you'll typically get offered up to 7 years and 100,000 miles. If you buy a car when it hits against 36,000 miles and three years (at the end of the bumper to bumper), you'll typically get offered up to 4 years / 60,000 miles.
One more note. GMPP rates increase steadily depending on when you buy it. They know people often discount the future and wait until the last minute. Buying a GMPP when your car is less than a year old could save you up to 50% on the price.
Edit: You can buy GMPP after the bumper-to-bumper expires, but the $0 deductible becomes a $50 deductible. Also the cost of the plan goes up and duration/miles starts to sink.
Unfortunately, I bought a 2 year old G6 with 33,000 miles on it, so I'm looking at $2,200 for 4 years 60,000 miles of Major Guard coverage. I'll be bidding out a couple more dealers to be sure over the next week though.