Hi Bellini
Welcome to Rootschat and especially the Sussex Boards - one of the best boards on line
Have checked the 1841 census for your William Gain in Westfield - unfortunately there is not one.
There are three seperate entries for the Gain Surname
1. George Gain, 20, Male Servant with Sidney Martin, Farmer.
2. Mercy Gain, 13, living with William Turner, Ag Lab & Family
3. John Gain, 17, Ag Lab with the Simon Foster, Ag Lab & Family
But I did notice this
HO107/ 1105/ 06/ Kent Street House, Westfield
GAINS William, 25, Ag Lab
......... Caroline,20
......... Henry, 1
All born in County of Sussex
Hope that helps
Chris in 1066