John (b. c. 1820, Blacksmith, and William (b. c. 1822), Coachman, may have been brothers, but I can't find any proof for you.
There is only one Gallop family listed as living in the actual area of Long Ashton, Somerset in 1841.This family is headed up by Samuel (1788-1860, Schoolmaster)and Hester Gallop (1787-after 1851, Schoolmistress). In 1841 they have several children living at home, but there is a gap between a couple of older ones aged 25 and some younger ones going down from age 15.This may imply that the older teenagers were off apprenticed or working elsewhere.One daughter, Elizabeth, appears to have been working in the area and this would have been a logical time for John to have been away as an apprentice as he has no Gallop family members in the neighbourhood that he could have apprenticed with (perhaps his mother's family?). There may have been an additional son, James, who joined the military. One of Samuel's sons, Edward, is listed as a "Coachbuilder's apprentice" so it is possible that William was also trained in this trade, but the William listed as Samuel's son is only 13.There is an 85 year old woman living with them, Sarah Noble, possibly a mother-in-law and two young children with the last name of Weeks born outside Somerset. The 1841 census is a tough one to work with because the ages were very often incorrect, even when not rounded off.There were also many men away at sea at the time of the 1841 census, so any fishermen or sailing men were not accounted for.
Anyways, Samuel and Hester are definitely not the John and Ann that you named as possible parents for John Gallop b. 1822, but there is no one else in the Long Ashton area to consider. William did not name any of his children Samuel, and that would be unusual for the time, so perhaps Samuel and Hester were auntie and uncle... I just can't imagine they aren't quite directly related in some way.
I don't have a John Gallop born around 1820 in Somerset living/dying in Victoria or NSW. They arrived on the Ticonderoga, but did they continue on to New Zealand? I have tried to record all the Gallops in Australia so I am interested to know if I've missed these ones.
As I was looking at the different records, particularly later records, it is clear that you will have to search a wide area to find these ancestors as similar locations are recorded as Long Ashton, Bedminster, Bristol, Gloucestershire, Somerset, Avon and so on as the Bristol area straddled three counties depending on the time period and records kept. I can email you the notes I have of all the Gallops in the Long Ashton area that I have info on (there's a few in the workhouse as well) if you are interested.Email me at my address firstname.lastname@example.org and I will send it direct to you. Good luck with your research.