gistfile1.txt

centauri :: ~/test » cat x.txt 
lkdjafs
lsjdkflkasjdf;lkajsdf
ajdsf
akjsdf
dsjflkasdfj

Installed Groups

a
b
c

Available Groups

d
e
f

group_names.rb

require 'facter'

Facter.add("group_names") do
  setcode do
    file = "x.txt"
    start = false
    finish = false
    data = []
    return nil unless File.exists?(file)
    File.read(file).each_line do |l|
      finish = true if l =~ /^Available Groups/
      data << l.chomp if start and not finish
      start = true if l =~ /^Installed Groups/
    end
    require 'pp'
    data.reject { |item| item.empty? }.join(",")
  end
end

output

centauri :: ~/test » facter group_names
a,b,c