The amount of life gained is equal to the destroyed creature’s toughness as it last existed on the battlefield. If it wasn’t destroyed (most likely because it has indestructible), you gain life equal to its toughness as it currently exists on the battlefield.

If the target creature becomes an illegal target for Vraska’s Stoneglare, the spell doesn’t resolve. You don’t gain life or search for Vraska, Regal Gorgon.