When you try to update you gallery using the WordPress Gallery Plugin, are you getting a string of php errors reading something alone the lines of: “Warning: array_merge() [function.array-merge]: Argument #1 is not an array?” If that’s the case, there may be a simple solution.
1. Exit the gallery post.
2. Reload the post.
3. Make sure the post has a title (This is the important step that you may be missing. Even the gallery needs a title! Try adding text content as well!)
4. Click “Update” or “Publish.”
-Don’t upload more than probably 8 or 10 photos at a time, especially if they are large. Wait for them to completely load, then click publish or update.
Your problem should be solved! Let me know in the comments if this helped you, or if you are still having trouble. I encountered this problem running WordPress 3.5.0.