When configuring a map gallery, provide an option for hiding specific Items.
Meaning provide a list of the Items that are being shown based upon the "Only display items of this type" setting and provide a check box next to each Items name. This would allow the person configuring the app to uncheck the boxes for the Items they don't want to show. Ideally the list would be grouped by Item type/app type.
In my opinion, having the web app show all of the Items within a given type by default, and then allowing the user to manually hide specific items as needed would resolve the issues described in the two use case examples below:
I have a number of web maps and apps which I want to show in the the same gallery (see this Idea about selecting multiple Item types), however, included in there are web maps that exist solely to be consumed by one of the Web Apps (of course, for keeping track of things both Items have nearly identical names).
The way I am using Map Galleries is to present a simplified view of important/frequent Items for less technical users. I don't want my users to select the web map that's being consumed and then be confused as to why it doesn't have the fancy UI (web mapping application) that they expected.
When I am showing web app Items, it shows both Web Mapping Applications as well as Survey123 apps. In many cases the Survey123 form utilizes URL parameters to pre-populate aspects of the survey when a link is clicked in a web map/app. I don't want the users to launch the survey from the Map Gallery, I need them to go into the Web Mapping Application (or possibly a web map) and launch the survey from the pop-up hyperlink. If I could hide/de-select the specific survey Items that utilize URL parameters this would resolve this problem for me.