If I have a 15mpg vehicle, I'm using 6.66 gallons to go 100 miles.
If I move that up to 18mpg, it takes 5.55 gallons to go that same 100 miles.
That means I've saved 1.11 gallons.

If I have a 30mpg vehicle, I'm using 3.33 gallons to go 100 miles.
If I move that up to 40mpg, it takes 2.5 gallons to go 100 miles.
That means I've saved 0.83 gallons.

So Double-Trinity is right, hybridizing the guzzler will save more gas.

However, I suspect that GM's motivation is to assuage customers. "It's okay to keep driving that 7-seater SUV, single guy. We'll make it a hybrid and you can pat yourself on the back. Emissions standards? Those don't matter."