Built in gas ovens almost always require a flexible hose. How on earth can they be fitted otherwise? The unit they are built into usually goes right back to the wall. The only room there is to connect it to a gas supply is from the 150mm space underneath the oven housing. As that's about 500mm deep then the gas fitter would have to have long arms, mirrors and the patience of a bloody saint if he was to fit it using a rigid pipe.
All those on here who say they must be fitted with a rigid pipe are talking the proverbial.