Actually, I lie, this patch only sort of works. The issue is that it 
only works when a ruby package is also installed, GEM_PATH does not get 
set as part of the ruby-build-system for yaggo. The lib/ files are 
copied to what I gather is the correct place, but the env isn't right.

The same also appears to be true of other ruby packages e.g. ruby-hoe:
$ guix package -i ruby-hoe
..
ben@u:~/git/guix$ guix package --search-paths
warning: failed to install locale: Invalid argument
export PATH="/home/ben/.guix-profile/bin:/home/ben/.guix-profile/sbin"
export PYTHONPATH="/home/ben/.guix-profile/lib/python3.4/site-packages"
export 
PKG_CONFIG_PATH="/home/ben/.guix-profile/lib/pkgconfig:/home/ben/.guix-profile/share/pkgconfig"
export GUILE_LOAD_PATH="/home/ben/.guix-profile/share/guile/site/2.0"
export 
GUILE_LOAD_COMPILED_PATH="/home/ben/.guix-profile/share/guile/site/2.0"
export CPATH="/home/ben/.guix-profile/include"
export LIBRARY_PATH="/home/ben/.guix-profile/lib"
export ACLOCAL_PATH="/home/ben/.guix-profile/share/aclocal"

i.e. no GEM_PATH entry, hence:
$ sow -h
/gnu/store/9iifw37m8vd5bkj0fh67ndc5f2da46wb-ruby-2.2.2/lib/ruby/2.2.0/rubygems/dependency.rb:315:in 
`to_specs': Could not find 'hoe' (>= 0) among 9 total gem(s) 
(Gem::LoadError)
Checked in 
'GEM_PATH=/home/ben/.gem/ruby/2.2.0:/gnu/store/9iifw37m8vd5bkj0fh67ndc5f2da46wb-ruby-2.2.2/lib/ruby/gems/2.2.0', 
execute `gem env` for more information
     from 
/gnu/store/9iifw37m8vd5bkj0fh67ndc5f2da46wb-ruby-2.2.2/lib/ruby/2.2.0/rubygems/dependency.rb:324:in 
`to_spec'
     from 
/gnu/store/9iifw37m8vd5bkj0fh67ndc5f2da46wb-ruby-2.2.2/lib/ruby/2.2.0/rubygems/core_ext/kernel_gem.rb:64:in 
`gem'
     from /home/ben/.guix-profile/bin/sow:22:in `<main>'

Files are there though:
$ ls /home/ben/.guix-profile/lib/ruby/gems/2.2.0/gems/
hoe-3.13.1  yaggo-1.5.4

FYI: I actually only care about yaggo because it is a build dependency 
of jellyfish, but I'm having some problems with make check so no patch yet.

Thanks,
ben