Evidence from genealogical sources prove the Dennis family came to Utah in 1855. They had a child born in Iowa Sept. 1854. The parents had temple work done in the Endowment House Nov. 1855. So they must have come on that season's emigration in 1855.
It is possible that the Dennis family traveled with the Gilbert & Gerrish company in 1855, but this possibility still requires verification.
His name is listed as "William F. Dennis" on the 1860 Utah Census index.