Unually involves removing the old jacuzzi and installing a new jacuzzi. Plus plumbing the Gas service to the location of the hot tub.

Replacing the guts of something usually isn't cheaper than replacing the item itself.

Usually not practical.

I have done it to a unit when the electric heater failed and the owner wanted gas. LP and NG are both far more efficient at heating a hot tub and the plumbing wasn't that difficult. Installed an external gas fired heater. MiniMax 100,000 BTU Heater - Millivolt (Natural Gas) [BW-NE970] - $811.38 : Kiddie Toys, Trampolines, Bounce Houses, Pedal Cars and Educational Games! (http://www.kiddietoys.com/minimax-100000-btu-heater-millivolt-natural-gas-p-7618.html?src=base&language=en) The install ran him over $1300 so you may want to think this over