There was a Gabriel Withers, age 23, carpenter who arrived in New York in 1847.
From New York Passenger Lists, 1820-1957
Name: Gabriel Withers
Arrival Date: 2 Dec 1847
Estimated Birth Year: abt 1822
Age: 25
Gender: Male
Port of Departure: London, England
Ship Name: Gladiator
Port of Arrival: New York, New York
Line: 31
Microfilm Serial: M237
Microfilm Roll: M237_70
List Number: 960
I wasn't able to find him on the US census.
Regarding John and Amy Mellor, it appears that the family came over at different times. I have found entries in the Boston Passenger Lists, 1820-1943 that match many of the children mentioned on the 1871 census but some arrived in 1871 and others in 1873. Some of the entries are confusing (from the 1873 lists) so I don't know if they are the correct family.
Name: Amy Mellor
Arrival Date: 9 May 1871
Age: 18 years
Estimated Birth Year: abt 1853
Gender: Female
Port of Departure: Liverpool, England
Ship Name: Siberia
Port of Arrival: Boston, Massachusetts
Microfilm Roll Number: M277_80
Listed with Amy were Theodore (age 9), Mary Ann (age 6), Charles W (age 5) and Sarah Jane (age 1).
Name: Amy Meller
Arrival Date: 31 Jul 1873
Age: 46 years
Estimated Birth Year: abt 1827
Gender: Female
Port of Departure: Queenstown, Ireland and Liverpool, England
Ship Name: Siberia
Port of Arrival: Boston, Massachusetts
Microfilm Roll Number: M277_85
Listed with Amy were Annie (age 13), James (age 10) and Elizabeth (age 6). Listed on a different page were Joseph (age 4) and Albt C (age 1). On the page before the one with Amy was an entry for a John Meller although the information listed for him doesn't fit (age 55, born: Ireland, occupation: brewer). No other entries came close to matching John.
I found John and Amy Mellor on the 1880 census in Utah:
Name: Emy Mellor
Home in 1880: Fayette, Sanpete, Utah
Age: 57
Estimated Birth Year: abt 1823
Birthplace: England
Relation to Head of Household: Wife
Spouse's Name: John
Father's birthplace: England
Mother's birthplace: England
Occupation: Keepshouse
Marital Status: Married
Race: White
Gender: Female
The rest of the household consisted of:
John Mellor, age 69, labourer, born: England
Emy Mellor 57
Edwin J. Mellor, age 17, labourer, born: England
Joseph Mellor, age 11, born: England
Albert C. Mellor, age 9, born: England
Marnie Mellor, age 19, born: England
Note: Most of the 1890 US census was destroyed in a fire. Information for Utah did not suvive.
Utah Burials -
http://history.utah.gov/apps/burials/execute/searchburialsName: Amy Bellamy Mellor
Gender: F (Female)
Birth Date: 7 May 1824
Death Date: 3 Nov 1891
Cemetery: Fayette Cemetery
Source: Sexton Records / Grant
Grave Location: 06-08-04
Relatives: Spouse Mellor, John
Name: John Mellor
Gender: M (Male)
Birth Date: 10 Feb 1811
Death Date: 17 Nov 1892
Cemetery: Fayette Cemetery
Source: Sexton Records / Grant
Grave Location: 06-08-05
Relatives: Spouse Bellamy, Amy
There are quite a few Mellor burials in Utah so I only put Amy and John here. The link above leads to the Utah State History page of the State of Utah website.
Regarding Joseph Withers, I notice that in your post you have a middle initial of "E" but the marriage entry I saw in the FreeBMD had Joseph Henry Withers marrying Mary E. Miles in 1890. I saw a Mary E. Withers born abt 1863 in Leicestershire on the 1901 census in Ilkley, Yorkshire. She was a "manageress of hydro". I don't know if this was Joseph's wife or not as she was listed as being single. I also found this death in the FreeBMD:
Name: Joseph Henry Withers
Estimated Birth Year: abt 1866
Year of Registration: 1895
Quarter of Registration: Oct-Nov-Dec
Age at Death: 29
District: Leicester (1837-1974)
County: Leicestershire
Volume: 7a
Page: 135
I hope that helps.