nanny jan and GrahamSimons, many thanks for your replies and I'm so sorry it's taken em so long to get back to this (family illness).
Yes, the "ag lab" Gabriel from the 1841 census is the right one. He is also shown as a labourer on his death certificate. His widow did remarry, but not until she was elderly, long after her daughter married, so I don't think it's a stepfather's occupation. Thanks for the suggestions, though.
I have found no trace of him in the military records so far, so I have come to the conclusion that someone was telling porkies.