What type of spa are you thinking about?

The pre-packaged ones all use electric heaters.

If you are talking about a inground spa where the equipment is seperate then you could use gas, but the line you ran for your BBQ is probably not large enough, depending on length of pipe and btu's used you may need a 1 1/2" line or more. If you were to do this, you would want the equipment and the noise located away from this area.

Hmmm, this may not be easy then. I can run a 2" line to where we would put the tub, thats easy, the plumber is coming over on Tuesday. But I dont want anything below ground because we usually end up with five feet of snow on the ground. I will snow blow around the tub of course, but we dont want to be sitting in a cave. It doesnt seem like it is going to be easy to get gas.

If a freeze is going to ruin a gas heater, it will ruin an electric heater.

http://www.redneckpoolheater.com/

you can always try this heater.

Or this...

There's absolutely no way that the BBQ's small gas line on ounce pressure will fire up any gas fired pool/spa heater.
Gas heater heat exchangers will freeze in the bitter cold like you enjoy during your winters. You'll need to run the pump anytime the temp is at or below freezing. Look at a Len Gordon FC5
They cost a little over 100 each and will be the best money you ever spent on your application.

See ya,
Kelly