Not to sound rude here, but have you tried reading ANY of the gallery forum messages before you posted this message? 

There are 20 messages already in the forum that talk about this exact error message that you are asking about, and the answer is the same for all of them.  Or may I suggest reading the front page of the Gallery Project on the DNN site.

There are so many articles that cover this particular topic (both the navigation issue and regards to DNN 4.7) that I feel it is almost insulting to have to write a list of the articles that cover these topics.  One thing that should be noted, and it is clearly listed as the first items on the Gallery--The gallery is beta, it doesn't work well, and it doesn't work at all with DNN 4.6 or higher.

So start with the basic note that the gallery module is a beta and the fact the you shouldn't be running it in production, and if you get it to work good luck. http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/9/threadid/44525/scope/posts/Default.aspx

How about this article that says the gallery doesn't work on DNN 4.6 or higher
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/9/threadid/189622/scope/posts/Default.aspx

Here is another article that talks about how the gallery doesn't work on DNN 4.6 or higher
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/9/threadid/187044/scope/posts/Default.aspx

Here is someone who asked the same question
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/9/threadid/186284/scope/posts/Default.aspx

Here is another person who got the same error
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/9/threadid/188208/scope/posts/Default.aspx

And since I think I listed enough items, I refer you to one of the first posters about this question, in which I responsed with the technical details of the problem, how I fixed it in my version of the gallery, and some workaround you might want to try to get it to work.
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/9/threadid/183287/scope/posts/Default.aspx

Other than turning off friendlyurls which _may_ work (but will apparently break other modules), there is no solution to this problem other than to wait for the Gallery team to release a fix.  Oh, and I guess there is one other solution and that is to not put any images/media in the "root" album, but instead to create albums and put the images/media in those albums.