If you haven't looked at them already - the collection of indexed "Marriage Certificates" that lists "Gudet & Mumbel" has been digitized and available online at the Milwaukee Public Library website (digital collections). The marriage license and certificate are most likely for your John Gudex & Wilhelmina Mampel. It's easy to see why the indexer interpreted the "x" as a "t". But the clerk used a different stroke in two other words ending in "t".
(The search function at the library website is not very helpful, but the original index for this collection at the local genealogy society site is more flexible.)
BTW - there was a Mrs. Margaretha Mample who died in 1887 at the age of 87 yrs, and her death notices were in two of the local German daily papers. A Friedrich Zinn died in 1888 and his death notices reference a Mampel. These are the only two occurrences of that surname that I've indexed - and no occurrences of Gudex so far.
"Milwaukee's German Newspapers; an index of death notices and related items" (1844-1950).
Gary Rebholz, compiler/editor
Indexing Milwaukee's German newspapers continuously since 2007... helping family researchers find their ancestors among Milwaukee's 99%.