From what I hear, the proper way to use Gravity would be to hit your plants with it for one week after they stop widening, to harden them up. So like for me...use Hydroplex up until they stop widening, then stop Hydroplex and hit them with Gravity for one week. Other stuff in there too, but ya.
edit: I say "from what I hear" because I've used it inappropriately in the past, using it for the last few weeks of flowering. But from my trusty and knowledgable hydro guy-source, he says you hit your plants with it for just the one week to harden them up.