Daniel and Donald are effectively used as equivalent names in Scotland, so it's not unusual to find someone originally baptised as Donald turning up in a later record as Daniel, or vice versa. So your Daniel Donald was probably plain Donald when he was baptised.
I see from the 1841 census that he had sons named Charles, William, John and Benjamin, and that John was born in Scotland. There is a five-year gap between John and Benjamin. Were there possibly other children who died young?
Have you found him in any censuses later than 1851? Do you have places of birth for Charles and William, who were not born in Gloucestershire?