Gravity doesn't determine whether or not you will use a secondary. You only need a secondary if you are dry hopping or adding fruit. Even then, a lot of folks don't use a secondary they just dry hop, or add fruit, to the primary.
A secondary does help to clean up the solids in the beer, but the times i've used one, i tried to only transfer when the fermentation is finished (FG reached) and the flavor just needs to clean up a bit.