As a general rule of checking, three things; the battery, anything that is rubber (serpentine belt, tyres) and the fluids (oils, coolant).
Battery: Even if you disconnect the battery, it cannot retain all the juice inside forever and it will drain eventually. This will reduce its life.
Engine: Your various belts and wires can get corroded if not used over a long period; the serpentine belt can get corroded, some animal can chew up the wire or do damage when it's resting.
Your valves and cylinder walls might get corroded due to lack of any oil or movement inside.
Brakes: This happened to me personally where they lose their grip over time.
The above things can go bad, additionally your wheels can get jammed if not used for months, and if you are near a coastal area, your can can be prone to faster corrosion.
Electrical: Sometimes the interior electricals tend to develop a fault such as the power windows get jammed, electric seats.
AC Gas leak: Cars that are not used over a long time might cause ineffective cooling from the Air Conditioner and will need to be recharged.
Fluids: Most of the fluids, like engine oil/brake fluid, break down, they lose their ability to lubricate eventually though this takes much much longer like half a year or so; but still you will need to replace them.
Since yours is a CNG vehicle, check if the hoses are fine and have not developed a leakage. I would also advise to use petrol if it's going to be intermittent usage since the gas does not lubricate the engine component such as valves as much as petrol; this will not cause any serious harm but it's better to use petrol for a few miles initially and then switch to LPG.
A car is designed to be run like a horse, you cannot get a horse and make it stand all day, you don't buy a horse to pet it, they are meant to be ridden else they will develop problems (or whatever horses become if not ridden).