first, sorry for your loss..........do you have a filter on this tank that has cartridges? they grow a bacteria colony in them thats what makes a cycled tank cycled. or the gravel has a colony as well. if i were you and i wanted to keep a tank cycled, id syphon the gravel and keep it in, do a water change as usual and then youd be able to add fish right away.
if you empty the tank and clean the gravel, youll be losing the beneficial bacteria, and the filter will be turned off, so id think the bacteria in it would die off as well. so, if you take the whole tank down, youll be starting over.