A version of the public map gallery is now included as part of ArcGIS Online. For any group you have that is shared publicly, you can create a gallery from the group using one of two group templates. If the group contains web maps or web mapping applications, they will be displayed.
The watershed application you reference is not one of the out-of-the-box templates. However if you have an application running on your web server, you can add it as an item to your My Content, by adding an item of type "application" in the Add Item dialog. Then, share that item with everyone and also add it to your group. Then you can use the Share button on the Group page to create a gallery application, published to ArcGIS Online.
Hope this helps,
Mike
