The 1881 census shows Matilda was 43 therefore born around 1838. I looked for a birth of any Matilda born IOM in that period and only match to the four names was :
Matilda Killey b 29/5/1831 to Edward and Jane Corrin.
The names Gellin, Gellen and Gelling linked as one on the IOM.
William's MC in Braddan 1887 gives him as Gelling, a car driver, residence 8 North Quay, Douglas. His father Thomas Gelling a Market Man.
No sign of any Gormal or Gurnell on the IOM.
It is looking like there was no marriage between Thomas and Matilda. Did you find one ? What happened to Thomas ?