Actually, the Gravity knob corrresponds to Decay time – though of course, other controls will affect the percieved length of this time. The Feedback knob controls the amount of feedback around the reverberation structure, so while it isn't technically controlling the tail length of the reverb itself, it does give the impression of doing so, yes.
You are (likely, I haven't tested) correct that the reverb tail cannot be sounded for more than 1 minute with Mod Depth and Mod Rate at 43 and 59.
