there is currently no wagn gem. wagn is itself a rails application and is not yet ready to include in other rails apps, but there are plans in the works.
You've probably already seen the standard installation docs. On that page there is a link to Installing Dependencies. They may be helpful in figuring out your mysql2 and rmagick issues. If not, I would consult the installation pages for those gems.
On my running server I have ruby-1.9 and all the other as updated as CentOS 6 can be.
The issue with rmagick is normal since rmagick is not keeping up with the development of imagemagick.
wagn "should" use mini_magick e.g. or something else, OR you could downgrade imagemagick in order to use old rmagick.
Nonetheless I am running wagn without rmagick (outcommented in the Gemfile) and e.g. logo upload worked fine.
Are we talking gems (mini_magick vs. imagemagick)? One hint is to do 'gem install ' for gems that give you problems with bundle install. Gives better debugging info.
I think it may work to upload without rmagick, but it probably won't scale the images for different size options.