I'd more or less discounted the George GIBSONs in the IGI (with mother Mary and born in Scotland round about the right time) - especially noting Mary Ann's other children in the 1861 census (George Thos 9 b Scotland, and John 5, Joseph 3, Matthew 9 mths all b Durham).
It seems that the parents of the IGI George GIBSONs all remained in Scotland.
From the IGI:
1. George b 1850 in Barony Lanarkshire, parents William GIBSON and Mary LAIRD. People of those names were married in 1849 in Barony and parents of those names had children in Lanarkshire or Renfrewshire in 1850, 52, 54, 57, 59, 60, 62, 64, 66, 68, 70, 72.
2. George b 1853 in Falkirk Stirlingshire, parents David GIBSON and Mary NIMMO. People of those names were married in Falkirk in 1836 and parents of those names had children in Falkirk from 1840, 42, 47, 49, 51, 53, 55, 57.
3. George b 1853 in Lonmay Aberdeen, parents George GIBSON and Mary HUTCHISON. People of those names were married in 1852 (LDS submission) in Lonmay and parents of those names had children in Aberdeenshire or Banffshire from 1853, 54, 56, 58, 60, 62, 64, 66, 68, 70, 71, 73, 77 (most of these are extracted entries but some of are LDS submissions).
Let's hope the 1851 census does come online as promised and that Mary Ann was both married and in Scotland at the time!
JAP