These are all great suggestions which may be what you're looking for.
I had a similar problem where mislav-will_paginate installs and adds
this to environment.rb file:
config.gem 'mislav-will_paginate', :version => '~> x.x.x', :lib =>
'will_paginate', :source => 'http://gems.github.com'
I just removed the version hash key -- :version => '~> x.x.x' -- and
used this instead:
config.gem 'mislav-will_paginate', :lib => 'will_paginate', :source =>
Obviously, this won't work if you need to stick to a particular
version (I'm using 2.2.3), but it does prevent me from getting the
missing gem errors. As always, your mileage may vary.