If you want all the information of the gallery photo, I would just have them post the link to the gallery photo, cut out the middleman. As there is no way to embed all the information of the gallery photo in forums. You can insert a gallery image, but it will not contain all the data in Gallery.
Of course, I do not know the purpose of the forum in this context but what are you gaining with the forum topic? Gallery has the ability for users to comment directly on the gallery album/photo. Having your users go directly to the gallery album or photo may be the best way to curve any confusion and any duplication of data. Then in the forum, if you do need to add a relationship between a forum topic and gallery album/image, just post the gallery album or photo's URL.