How to use the final blast, or the revive

To change the gravity, you need to have the Gravity suit. Once you have it, press the screen 30 times (non-consecutive: therefore if you crash, your counter will continue where it left off next time you pick up that vehicle) and you will complete that mission. To get the Gravity suit, try to obtain the vehicle item in your current run.

The vehicle pick-up appears as a rainbow block with gears in it.

From my experience the spawn type has been random

When the player touches the screen gravity reverses and Barry walks whether on the ceiling or on the ground depending on the direction gravity is facing.

Source of quote and picture

I've had the same issue. I've found that sometimes my clothing changes to a previous setting, my money changes to previous times, and my rewards will disappear. Even though this is an older question, I really wanted to say that I had also experienced it. What I do to alleviate it is to make sure to go back to the Jetpack Joyride 'Home' screen and then push my device's 'Back' button until the program asks if I want to quit. This seems to work well enough.