I'm inclined to agree with the people who suggest adding more text. It's been my experience it helps. I've also read other semi-success stories here on WW from folks using the "add more text" approach.
I don't have any official way of estimating, however, based on my experience and reading about others' experiences here, I'd guess that maybe 85%-90% of the supplemental listings for photo galleries can be traced to lack of text on the page.
Think of the issue as one where a picture may be worth a thousand words, however, not to the googlebot. The Google guidelines say,
"Try to use text instead of images to display important names, content, or links. The Google crawler doesn't recognize text contained in images."
I'd also suggest double and triple checking your gallery's source code. It's also been my experience that a majority of open source photo galleries still run on code that produces too much duplicate content.
For example, you can have two or three different urls for one photo based on whether or not the software allows you to show different sizes of the same photo.
A general example of this might be
url.... (the photo as you want it to be seen by the public)
url&full=1 (for a larger photo)
url&full=2 (for an even larger photo)
Another problem that I've noticed with many open source photo galleries (at least the ones I've looked at on the open source comparison site (whose url will remain unsaid) is their lack of attention to properly dealing with deleted photos. In other words, they don't properly put out 404 or 410 error codes when you delete a photo, or an entire album.
One way to check for these potential problems, and others, is to carefully examine the source code as it looks when the gallery is called up in a browser. Typically there are many different urls present for navigation, thumbnails, etc. al.
If you do all the code rechecking and things look ok, then I would have more confidence saying that adding text to your photo pages will help you out of the supplemental index.