Gallery apps display thumbnails for all items within a group. At present, if a form is present in a group used by a gallery application, and someone tries to open the form, the form gets downloaded as a .zip file, rather than opening in a browser or the Survey 123 field app.
The user experience should be the same as when someone tries to open the survey directly from a group, the item's details page, or the featured gallery.
According to Esri, this is a feature request, rather than a bug. It appears that a workaround may be to create an “app” that points to the Survey’s URL, and to include the app in the gallery rather than the form. But this results in extra items, and sometimes groups, needing to be created and shared, which can create confusion for the survey owner and users. And I'm not sure that the workaround will work for both web-based and Connect-based forms.