I don't think U need gas to augment a solar array, if it is sized correctly for a given temp.
But do you want to go swimming and get out of a pool wet in 55 and under degrees, it's cold on your skin.
So if you design the panels to give you 80 water temp at 60-65 air temp it's very doable with solar. This design should include a solar cover.
That being said a gas heater is about $2,000 installed I think and if you're in an area without natural gas add the cost of a propane tank, how much will additional panels cost to do the same.
Of course with gas U increase you're
carbon footprint