Hit the wall--John Gittleson, Chicago

Hi, I just recently discovered the Norwegian branch of my family tree. It's been an interesting journey. My mom's father was mentally ill which my grandmother didn't know until after they were married. When she approached his family for help, they pretty much told her that he was her problem now and the relationship went downhill from there. As a result, there was a huge gap in our family history. I've been able to piece some of it together and through a google search, I discovered that my 4th great grandfather was Bard Jonson Lassehaug, one of the early settlers in Chicago. He came from Voss in 1837. His daughter Signey married John Gittleson, a sailmaker and businessman in Chicago. According to "History of Norwegians in Illinois", John was a member of the Norwegian Old Settlers Society and arrived in 1850 but there's no information on where he came from. I'm not quite sure where to look next, I'm fairly certain that Gittleson is the americanized spelling (because on my great grandmother's birth certificate her name is spelled Gettleson) so I'm hoping maybe someone here might have a better idea of where I should look next? Thanks, Michelle Smith