Fabulous 400 was offered in a gas version
According the ad in the link I posted above, both electric and gas versions of the Fabulous 400 were offered. The designs of the Flair and the Fabulous 400 are quite similar, but we don't really know if one manufacturer made them for both brands. The main difference I can see is that the Flair was free standing and included storage cabinets below the cooktop level. So it needed a 40" or so gap in the cabinetry. The 400 mounted on top of a base cabinet, in 30" or 40" versions, and the cabinetry beneath the unit matched the rest of the kitchen.
The Flair would definitely look like a freestanding unit in a typical kitchen, like the Hotpoint and GE ranges of the late 60s and early 70s that had an overhead conventional oven over the cooktop, and a larger oven below the cooktop, with integrated exhaust fan. The 400 would have had a semi-built-in look. It was exposed on its top and sides, but the bottom was attached directly to base cabinets, and the unit had no feet or base of its own.