Gallery 2 is an amazing collection of code. Wow! The deeper I look, the more impressed I become. While the documentation is very good [surprisingly good in fact], I have had to hunt quite a bit for answers. Below is a collection of links that I found helpful in my journeys.
WPG2 – WordPress Gallery 2 Plugin
http://wpg2.galleryembedded.com/index.php/Main_Page
WPG2 Themes
http://wpg2themes.galleryembedded.com/
WPG2 Themes Discussion
http://www.galleryembedded.com/forums/viewforum.php?f=12
How To Create A Theme
http://codex.gallery2.org/index.php/Gallery2:Themes:How_to_Create_a_New_Theme
How To Create A Color Pack
http://codex.gallery2.org/index.php/Gallery2:Modules:colorpack