Petter had a tendency to do that.
There is a reason that most Gnats are seen with two underwing tanks.
View attachment 577971
One story has the Gnat in contention with the Fiat G 91 for Nato light strike fighter.
One problem was the Gnats landing gear/tires were not suitable for the rough airstrip requirement, and larger tires requiring bulged landing gear doors were needed.
Petter refused to fit the larger tires and doors and Helped the G 91 win the competition.
Petter was reported to be not the easiest man to work with.
It sounds lucky that the area rule allowed the belly tank to be added to the English Electric Lightning with a reduction in supersonic drag.