Make sure to confirm with the author if it could fit your needs before a potential purchase.
If that doesn’t help, Gravity Forms areofficial partners withCodeable, who are the #1 WordPress outsourcing service. They can provide quality development from hand-picked Gravity Forms specialists for any project size and budget. Additionally, they can also provide you with a no obligation estimate of what the project may cost up front. You can reach them via this link.
You can also post to our Community Forum Job Board, We do not vet any of the individual developers there, it is an open forum for all, but our community is certainly full of talented folks!
My first guess will be that our GP Populate Anything Perk with some custom codes will work for you, but that will also depend on your exact use case. Can you confirm if the added city will be displayed as a choice in the dropdown field the next time the form is opened or will it be added to the dropdown choices for the current session? You can get in touch with us via our presale forms, so we can get more information on your use case and assist you further.