Looking at the 1865 census, Halsten Steffensen and his family are no longer living at Brathole, although they were still there when Gjertrud was confirmed in 1862. There is no Gjertrud Halstensdatter (or Steffensen) living in Voss parish. This probably means one of two things:
The entire family left the parish between 1862 and 1865 (probably for America).
Or Halsten and his wife Britha died or moved and Gjertrud went to America alone. Seeing as she was only 18 years old in 1865, I would believe her entire family went to America as she was probably too young to have reason to travel alone.
I can not find a record of the family leaving the Voss parish.
From what I undersdtand, your only reason for thinking that Mons Knudsen might have been from Rasdal is because you found that name of someone traveling. The death record for Mons Knudsen shows he was born in 1832. I think this is a better possibility:
#94, right page
bachelor, Mons Knutsen Tøssen, born 29 Jul 1832
leaving Voss parish for America on 7 Apr 1866
Kildeinformasjon: Hordaland fylke, Voss, Ministerialbok nr. A 20 (1855-1886), Utflyttede 1866, side 289.
Permanent sidelenke: http://www.arkivverket.no/URN:kb_read?idx_kildeid=7093&i...
I think there is a good chance that Gjertrud married someone she knew from Norway. This Mons might fit.