It caused about we new add it in the Beta 2, and we have done some issue on the first adding.  But it is fixed for the release. You can change the compatibleversions line of the Dotnetnuke.Gallery.dnn file

to

<compatibleversions>^[0]?[4-9]{1}\.[0-9]{1}([8]{1}\.[0-9]{1}[4-9]{1})|([9]{1}\.[0-9]{1}[0-9]{1})$</compatibleversions>
 

with the Editor.  Save it and add it to the zip folder.

I just had this problem as well. Looks like you need to update the beta download if possible.  I solved it by just deleting the < compatibleversions > line from the .dnn file.

JK.