The Gate can be visited
Fortress Mainz-Gonsenheimer Tor

The Gonsenheimer Tor in Mainz-Hartenberg-Münchfeld is a former fortress gate that was once part of the Mainz Fortress. It served as a sally port and secured the flood-proof routes to the west, an element of the Rheingauwall.
History+
Construction of the Gonsenheimer Tor began in 1872, shortly after the Franco-Prussian War, to protect the military expansion of the city of Mainz. By 1877, the complex was expanded with ramparts, a moat, and elevated positions. The gate, located at the highest topographical point of the Rheingauwall, was one of the few passages and lay between Fort Hartenberg and Kavalier Judensand. It featured separate passages for vehicles and pedestrians, as well as barrel-vaulted casemates illuminated from the city side.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Gonsenheimer_Tor.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 50.0036, 8.2476