Totally different types of products. Gravity is a flower hardener/ripener and is only used once or twice about 3 weeks before harvest.
Phosphorous loading products are used at different points in flower depending on what the bottle says... you have to read the bottle and follow instructions.
I use both Gravity and a P blast type product on mine. You MUST have your temperatures under control, and have plants in PERFECT health, to use Gravity. Otherwise you're likely to damage/herm/kill them.