sounds like it'll take some creativity on your part. look at cities you admire and try to recreate a history of their 'gateway' conditions.

probably the more fruitful places to learn from will be those in which there wasn't an explicit attempt to make a gateway so much as there is a condition of feeling like you've arrived. in cincinnati, it's when you come over a rise heading north and the city stretches out before you, just before you cross the bridge into downtown. in louisville it's either bridges or it's when, coming either west or south, you turn to follow the river into downtown. in manhattan it's often tunnels. in venice, historically, it was the place where the piazza touches the water by the ducal palace.



there is a typology of gate, exemplified by the tripartite pavilions like the various arcs de triomphe and the brandenburg gate. also the st louis arch, the gatehouses to various manors, and the gates of concentration camps like auschwitz - all fit the typological expectation.



for a very essential version, consider the gate as threshold: one side/other side. although it was diluted by there being so MANY gates, christo's gates project had some nice moments of threshold.

one year the steedman competition had a site in the center of paris, the quai de tournelle which overlooks the buttress end of notre dame cathedral. doing the competition i learned that the quai had been the original city gateway/toll, which kind of changes how you look at the site situation, etc.

If you also want some historical precedents outside of say the brandenburg and arc de triomphe, look at some of the medieval cities such as Volterra, or other walled cities in which they have gates to enter the city, and the relevance they might have to your project.