Palindr˜»me wrote:

But it would be fairly straightforward to do - without interfering with
the machine and/or invalidating its warranty. It's obviously not as good
as a machine taking both supplies and able to set its own temperature
for different cycles or parts of them.

But without changing things round inside the machine it will take in
hot water not just for the wash, but also for each rinse in the cycle
(I think there are usually three). Using hot water to rinse isn't
necessary, and the small amount you save from using gas-heated water to
wash in will be eaten up several times over by doing hot rinses.