I don't think so. But you can go to the top level of a folder or drive and press Ctrl-Shift-N to see its contents and that of all subfolders (or right-click on a drive in the tree and pick Include Child folders).
Or, you can use Search (Ctrl-F) to find all thumbnailed files in the database and save the results of this query to a Found List, which is very similar to a gallery. It will appear in the tree under Found Lists so you can easily return to it (and you can update it at any time by right-clicking on its name and using Requery Now).