I'm using the WPG2 plugin to link my Gallery2 install and WordPress together. Happily setting it up abd thinking it's all working, until one of my friends tells me he's getting a 404 when he clicks on the thumbnails WPG2 creates. Bit of investigation by setting up a test user, and it turns out this only happens if you are not logged in as an Admin or Editor. Contributors, Authors, Subscribers and those not logged in all get a 404 error.
Any info that might help, let me know and I'll try and provide it.
Also raised this on the G2 forum here: http://gallery.menalto.com/node/82414
Cross posting to Plugins and Hacks and How-To and Troubleshooting too. Hope that's ok.