The surname Gourdin (and variant Gourdain) is mostly found in the north of the country, as well as over the border in Belgium. However, unless we get lucky, it will be hard to search without a more exact location (there is no equivalent of freebmd for France).
French families often swap around first names (people will have baptismal names) so "Marc" at marriage and "William" to family is not unusual, e.g.:
official first names: Jean Pierre Olivier
normally went by: Olivier (or Oliver)
So yours may appear as William Mark/Mark William/Guillaume Marc/Marc Guillaume in various records.
Note: a son appears on Australia WWI records as "William Mark Gourdin" (father William), born Preston.
His birth appears to have been registered in Preston as "William Mark Gordon".
A second note on naturalisation status/"british subject" - this was often not accurately reported on the census. It might not be what he himself reported but that somebody assumed "Gordon" must be a British name when totting up statistics. I don't see any trace that he actually naturalised.