Wilhelm's father was Lars Larsen and his mother was Mathilde Kristofersdatter. Gjersvik was the name of the farm where they lived in the district of Hordaland, Norway known as Tysnes.

Gjersvik would have been their address and sometimes emigrants chose to use such an identifier as a 'surname' in the new country. Also in the 1920's Norway decided that families should CHOOSE a family name and no longer use the ancient traditional patronymic naming -- some chose to use their address/farm name - some chose to use some form of their old patronymic.

You can see the family in the 1910 Norwegian census -
http://www.rhd.uit.no/folketellinger/ftliste_e.aspx?ft=1910&...

Wilhelm's birth/baptismal record in the Tysnes church records is found here -
http://www.arkivverket.no/URN:kb_read?idx_kildeid=4532&i...

Online study material to learn about Norwegian naming practices and patterns:

http://homepages.rootsweb.ancestry.com/~norway/na12.html
http://www.nndata.no/home/jborgos/names.htm
http://www.norwayheritage.com/norwegian-names.htm
http://www.nndata.no/home/jborgos/farms.htm

The 1910 census above includes Mathide's widowed father -- Kristofer Olsen. Then taking the identifying information about him from that enumeration it is possible to find him in the 1875 Norwegian census --
http://www.rhd.uit.no/folketellinger/ftliste_e.aspx?ft=1875&...

Wilhelm was giving his patronymic name as Larsen when he sailed from Bergen in 1921 but he also uses Gjersvik as an additional identifier that was requested by the police collecting the names of emigrants -
http://digitalarkivet.uib.no/cgi-win/webcens.exe?slag=visbas...