You could put the ohai plugin file in /cookbooks/ohai/files/default/plugins and the ohai cookbook will distribute it for you.

gamespy.ohai.erb

provides "gamespy"

require_plugin "kernel"
require_plugin "network"

def get_ip_address(name, eth)
  network[:interfaces][eth][:addresses].each do |key, info|
    gamespy[name] = key if info['family'] == 'inet'
  end
end

def has_dsr_interfaces?
  return network[:interfaces].has_key?('eth0') &&
         network[:interfaces].has_key?('eth1') &&
         network[:interfaces].has_key?('lo:0') 
end

def looks_like_matchmaking?
  has_dsr_interfaces?
end

if looks_like_matchmaking?
  gamespy Mash.new
  
  get_ip_address(:private_ipv4, :eth0)
  get_ip_address(:public_ipv4, :eth1)
  get_ip_address(:loopback, "lo:0".to_sym)
end

gamespy.rb

# this code is not needed if use the ohai cookbook to distribute plugin

# force template creation and ohai reload in compile phase so 
# the attribute data returned by the plugin data is available for 
# recipes during execute phase

template "#{node[:ohai][:plugin_path]}/gamespy.rb" do
  source "gamespy.ohai.erb"
  owner "root"
  group "root"
  mode 0755
  action :nothing
end.run_action(:create)

ohai 'gamespy' do
  action :nothing
end.run_action(:reload)