Even larger heat pumps can be sized for a #8 THWN circuit. The units only require :Jandy 137Mbtu 42A branch circuit and Hayward 140Mbtu a 36A branch circuit.
https://www.hayward-pool.com/pdf/literature/HeatPro-Brochure13.pdf
It's fairly simple to compare fuel consumption rates, if you know what model equipment and your local rates.
For heat pumps, COP rating is a direct multiple relationship to power vs electrical btuh value.
So, if 1kWh of electricity costs $0.12, your Hayward unit is rated at 6.0 COP, and your output is 140Mbtuh, then you can calculate
140,000/3412/6x0.12= $0.82 per hr of operation or per 140,000 btus.
Gas supplier btuh value may differ but average is 1030 btuh/cuft.
So, if 1CCF (100cuft) of gas costs $1.18, the unit efficiency is ~83%, and the input is 200Mbtuh.
So, 200,000/103,000 x 1.18 = $2.29 per hr of operation.
But wait, it's only 83% efficient so, 200,000 x .83 = 160,000btus output.
So, you paid $2.29 for 160,000 btus in an hr of operation.
So, cost wise, there is absolutely no comparison. Heat pump wins hands down 59% more efficient using my local utility cost. Of course this is rated at an 80°/80°/80% temp. So, efficiency will vary with water, outdoor air temp and relative humidity, but still not even close to a comparison.
Time it takes to heat the pool, a big + for gas because of the heating value available.
Edit, just noticed I was looking at specs from 2013.
Here are 2015 specs:
http://www.hayward-pool.com/pdf/literature/heatpro-brochure-lithtpro15.pdf
So, your min amp circuit rating is 42A with the 2015 model. So, 50A circuit is plenty in either case, with a 60A breaker.
For those who may disagree, this is a multi motor load with internal overloads built in. Read Article 430 NEC, Circuit size is not rated the same as other loads.