| | Re: converting my non planted tank
Hornwort (ceratophyllum), java fern, and elodea come to mind. While generally not recommended, I've never had a problem rooting sword plants into gravel and they've always done very well. Java fern's supposed to go on wood or stone, but I placed some of mine onto the gravel and it's rooted in after a couple of weeks and starting to creep along the gravel surface.Just don't bury it, its roots should be exposed to the open water.
In fact, call me a renegade, but I think it must be possible to grow just about any aquatic plant in gravel. Considering hydroponics depends on growing terrestrial plants in inert media and weak fertilizer, then the same should be feasible for aquatic plants. Consider something I've just read about, portable farming, which utilizes fish waste as the sole means for sustaining a hydroponics system.
Then again, you didn't mention anything about fish. If you have no fish, in particular dirty fish, then never mind what I say because my experience depends on their presence.