[pushkins]

There are a few problems your going to face.
1. Foam is going to give you around a max of 3.3 - 4.5 R / inch so you might get R8-R9 value.
2. The foam will deteriorate rapidly unless it is covered, so gluing to to your existing drywall will be a very temporary measure.

The biggest issue is going to be code violation. Code doesn't allow foam insulation to be left uncovered in a garage, ESPECIALLY with living space above (your bedrooms).
You could however glue the foam insulation to the drywall and then apply a second layer of drywall over the top to encapsulate it. Min. thickness of drywall in a garage with attached living space is 5/8".

Were it me, I'd pull down the existing drywall in a small section, look to see what R value insulation is in place. If it is less than R19 then I'd be looking at replacing the insulation with R19 and re drywalling the ceiling.

Another option would be to look at the insulation in the walls of the garage.