I was trying to setup the Beta 2 from www.dnnmediagallery.com, on the latest DNN release (04.09.00). The installation appears to go well until the very end, where I was getting:

I've tried the Beta1 as well; same thing. You can read the entire installation log further down.

Anyway, it appears that the problem is in the one regular expression in the .dnn file. To get the installation going (whether the beta works or not, that's a different story), you should extract the Dotnetnuke.Modules.Gallery.dnn file, open it up in a text editor, and change the "4" in the following line to read "0". Then zip it back in (overwriting the original). That's it folks, it worked in my case!

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