Since this works for everyone...
If you've inserted the images in your posts via the Gallery function, you can pull the Gallery in on another page or post by using the PostID# of the original post where the Gallery appears.
If you mean mix up the images from several posts into a new Gallery, you can do this using the "include" parameter and the image ID#
In both cases you will have to write the Gallery shortcode and it's parameters in the HTML editor.
Very Large Caveat: In order to avoid navigation problems, include 'link="file"' in your Gallery shortcode, which will force the thumbnail to open the larger image directly rather than in an Attachment page. The reason is that if you use an Attachment page, it will bring the viewer to the original Post/Gallery where the photo was uploaded and inserted