Find greatest common divisor - ruby class (Euclid algorithm)

gcd.rb

# Finds greatest common divisor from 2 digits parsed from console
# Usage: ruby gcd.rb 5 2
# Finds the greatest common divisor from the digits 5 and 2

class GCD
  $digits = Array.new

  def initialize(args)
    if args.length == 2

      args.each do |a|
        if a.match(/^\d+$/)
          $digits.push(a.to_i)
        else
          error_message(2)
          return 2
        end
      end

    else
      error_message(1)
      return 1
    end

    find_gcd($digits)
  end

  def find_gcd(digits)
    a = digits[0]
    b = digits[1]

    if b == 0
      print 'The greatest common divisor for ' + $digits[0].to_s + ' and ' + $digits[1].to_s + ' is: '
      print a.to_s
    else
      if a > b
        a = a - b
        find_gcd([a, b])
      elsif a <= b
        b = b - a
        find_gcd([a, b])
      end
    end
  end

  def error_message(id)
    print 'Error: '

    case id
      when 1 # The amount of arguments is less or more than 2
        print 'Please parse 2 integers as arguments.'

      when 2 # One of the arguments is not a valid digit
        print 'One of the arguments is not a number.'

      else print 'an undefined error has occurred...'
    end
  end
end

main = GCD.new(ARGV)