This is what I did

This is the .rb file: gxg_bootstrap.rb
________________________________________________________________________
# Require Core Library:
require 'opal/full'
# Require Standard Library:
require 'benchmark'
require 'bigdecimal'
require 'buffer'
require 'base64'
require 'console'
require 'date'
require 'delegate'
require 'enumerator'
require 'erb'
require 'fileutils'
require 'forwardable'
require 'iconv'
require 'json'
require 'js'
require 'observer'
require 'opal-parser'
require 'opal-platform'
require 'ostruct'
require 'pathname'
require 'pp'
require 'promise'
require 'securerandom'
require 'set'
require 'singleton'
require 'stringio'
require 'strscan'
require 'template'
require 'thread'
require 'time'
# opal-browser Integration:
require 'opal-browser'

This is how I compiled it (with local symlink pointing to opal-browser gem dir):
________________________________________________________________________________
opal -I ./opal-browser/opal --gem paggio --compile gxg_bootstrap.rb > ../lib/public/external/gxg_bootstrap.js

Also note: when I just did a compile with '--gem opal-browser' instead of including the two dirs, I'd get 'add_method' undefined for Opal in the Browser.