A General Aviation pilot reported entering CHD Class D airspace at 2;900 feet on a training flight with an instructor; citing the smell of a gas leak as a contributing factor.

Narrative

This was a training flight. The subject airplane took off from ZZZ and headed to the south. When [we] overflew CHD at 3;200 ft. MSL; both the pilot and instructor; who [was] seated in the back seat smelled gasoline; the pilot decided to return to ZZZ. During turning to the north; heading 020; the pilot was occupied by the gas leakage and did not maintain the altitude above CHD Class D ceiling (3;000 ft. MSL). The subject airplane descended to 2;900 ft. MSL for a few seconds. The instructor immediately warned the pilot regarding 3;000 ft. MSL for CHD Class D ceiling. The pilot applied full power and climbing to 3;200 ft. MSL. Total time period for violating CHD Class D airspace was less around 15 seconds.After landing [at] ZZZ; the pilot found the gasoline smelling was [coming] from the right wing-tank. The vent tube of the fuel tank cap was pointed to the rear. The correct position of the vent tube should be pointed to the front. The pilot fueled up the fuel tank prior to the flight. It was a pilot's negligent of not correctly tightening the fuel cap.There were two lessons learned; 1) when encountering difficulty in a flight; the pilot in command should first 'fly the airplane'. 2) the pilot in command must get familiar with the airplane.