2 scenarios for consideration:
1. "My Organization's Featured Content" seems to be the default using Portal for ArcGIS 10.6. What version are you using?
2. The Maps and Apps Gallery is an application that accesses a collection of maps, apps, and other content cataloged in a group. You can set which Group you would like the Gallery to show by going to My Organization > Edit Settings > Gallery tab. You won't be able to actually replace this tab with the application.