The line I'm researching was headed by a Heinrich Tilmann Gassen, and emigrated from Köln area to Wisconsin in a couple of groups from 1842 through about 1845; part of the family staying in the Milwaukee County area (Towns of Oak Creek and Lake) and another part moving to the west side of the state to Grant, Iowa, and Richland Counties by 1860.
I have some hints of connections to Iowa, but also the birth of a [Gassen & Dederichs] in Detroit c. 1843.
I've been in touch with other Gassen researchers who are part of the Missouri branch. There is a possible connection there with an 1860 emigration to the Verona Twp., Dane County, Wis. area by a Herman Gassen.
Happy to correspond further via email with details of the outlined info above.