I have tried NextGEN Gallery and Lazyest Gallery, but both seem to require to upload it with their form. PhotoSmash is not supported any longer.
Could you please tell me which plugin (that is supported and works with the latest WordPress) can generate galleries with already uploaded photos?
Lazyest Gallery does not require to upload using the admin form.
In fact, the plugin is built to automatically index your existing uploaded images.
Just enter your images folder in the lazyest gallery settings and the plugin will build a gallery.
(I am the author of Lazyest Gallery)