Hello @Kris Van Daele
Welcome to Microsoft Q&A Platform,
Application Gateway is a multisite product so you can add manage the sites in a single gateway.
This will require to replicate the settings (frontends, backends, routing rules, etc...) from the gateway that will be deleted to the one that will remain active.
https://learn.microsoft.com/en-us/azure/application-gateway/multiple-site-overview
https://learn.microsoft.com/en-us/azure/application-gateway/create-multiple-sites-portal
I hope this helps!
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