This can be done but I dont recommend it. First a concrete spa has poor insulation. They don't work anywhere close to what a standalone spa does for hydrotherapy, again not even close.
2nd the cost of gas heater is going to be hundreds each month.
3rd, if we all learned something from the great freeze of 2021 is pools and spa in freezing temps do not work.. You have no way to ensure you can keep power to the spa in freezing temps. In water temps colder than 68 degrees you can not (are not supposed to or you will damage heater) run the gas heater, so there will be no way to heat the spa in water temps colder than 68 degrees...
From your manual:
View attachment 613640
From Pentair manual, but applies to all gas heaters:
View attachment 613641
4th, you have to be very careful to not accidentally allow water past a valve to the pool and freeze damage pipes. IIRC, your pool and spa are on the same pump...then this is an issue.
5th, you would need a generator for power failure, and is great, but what happens if pump goes bad or heater dies a year or a few down the road in middle of winter?
For probably less money a luxury hot tub can be placed that will cost you less than 50 a month in dead if winter, 10 max I summer, and is ready to go at 100+ degrees 24/7. And if it dies you have days before water freezes, even then a small heater in equipment bay can buy you lots of time.