What is the best way I can use App gateway for multiple applications ?

You'd configure your applications to all live under separate domains, subdomains or paths behind your application gateway. Consider having WordPress CMS, but using it headlessly and having a front end written in React that can consume the WordPress API. You'll now have a powerful CMS, with no annual licensing fees that can be extended in more ways than I can explain and is already more supported than most other CMSs out there and you can easily, effectively share them via 1 domain name. You can expose your content via an endpoint and mask the fact that you're using WordPress and call that content endpoint or path from your front-end and the Gayeway will manage the routing, the security, you can lock your actual cms backends admin path down and pretty much obscure the fact that you're using WordPress entirely from anyone who may be crawling your website which provides even more security.