We have galleries that retrieve their data from SharePoint lists and also use the AddColumns function to bring in related data. It works great, but can take a number of seconds to retrieve the data. It's not a great user experience as the gallery shows as blank during this time. We have tried using a label on the screen based on CountRows of the gallery, but this stays being displayed if no records are returned, so this method is no good.
It would be great if there was some custom "loading" text we could set on the gallery that is displayed whilst waiting for the data to be displayed.