jeff wrote:

can they be installed on gas water heaters?

Sure. Looks like they have even less insulation, R5.6 vs R11.5 for
Grainger's Rheem versions. The blanket savings would be greater, with
a faster payback, but it's likely to be a smaller proportion of the total
heat loss, given the large open flue up the middle, unless there's
a gadget that closes the fluepipe when the heater stops running.

Nick