@Raviraj Nagenhatti , The difference between the Gallery Apps and Non-Gallery Apps are, Microsoft have worked with a lot of third party apps for eg: SalesForce, ServiceNow etc, and have added them to the Azure Gallery Apps platform. By adding we mean, we have done the basic integration and has made most of the generic settings already pre-created in Azure for the users, where as, in non-galley apps only the basic skeleton for adding an application is provided, and are mostly preferred incase you plan to integrate a SAML app that you are developing or some app that is not present in the Gallery Apps list of available apps.
But we always recommend to initially try to look for a SAML app if that is already present in the Gallery App section or not, if not then only proceed with the Non-Gallery App integration.
Hope this helps.
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