Matt, One of the quick hacks to get the previous version of the Gallery in install or work once installed (I can't remember which) was to remove the DNN_ prefix.  That is what is causing your problems now.  When you go to install the new version it would upgrade the module but there is no longer a module named DNN_Gallery so it goes to do a normal install.  There it finds that in the database there is already a module with the friendly name of Gallery that is a unique key and fails.

Just backup your Gallery directory just in case, uninstall the old version and install the new one.  Then go back into the Gallery folder and copy your back up into the new directory that the new Gallery just installed.  You should probably run maintenance after all that to get the files synced back up and you should be good to go.

I originally had the 3.00.10 beta version of the Gallery installed on DNN 3.0.11 release and all was good.  Then DNN 3.1 was released and instead of doing an upgrade since I had not gone live with anything I did a complete reinstall of DNN 3.1. Here is where things started breaking for me and a lot of others.  As I understand it from reading other posts here in DNN 3.1 modules needed to start with DNN_ (or not its hard to piece together).  Anyway the DNN 3.1 install broke the Gallery 3.00.10 install and the fix was to change the Module name from DNN_Gallery to Gallery.  Install works.

Upgrading the from DNN 3.1 through 3.2.1 Gallery version 3.00.10 Gallery works if you don't look at it funny or reinstall it.  Now running 3.2.1 upgrading Gallery to 3.0.11 from .10 install errors on a unique key in the DB.  The only unique key that could be causing a problem is the friendly name.  And the module won't upgrade the previous version because the module name had to be changed from DNN_Gallery in version 3.0.10 to Gallery to get it to install with version 3.1+ DNN.

These are the combinations I've tried in any case.