This code snippet assumes you are using the Rack style bootstrapping as detailed in Gollum's Readme. This code must go AFTER the require libraries and before any objects are first created.

gollum.rb

##
# This is an example of how one might go about monkey-patching custom syntax
# into the gollum wiki engine. 
##

# Monkey Patch the gollum library to support {.css-class} syntax
# Mechanics derived from http://stackoverflow.com/questions/4470108/when-monkey-patching-a-method-can-you-call-the-overridden-method-from-the-new-i
class Gollum::Markup

    # Add our class variable, cssmap.
    old_initialize = instance_method(:initialize)
    define_method(:initialize) do |page|
        old_initialize.bind(self).call(page)
        @cssmap = {}
    end

    # Subvert the render method, wrap it with our two calls, extract and process
    # Definitely one of those times I miss Moose.
    old_render = instance_method(:render)
    define_method(:render) do |no_follow,encoding|
        orig_data = @data.dup
        @data = extract_css(@data.dup)
        doc = old_render.bind(self).call(no_follow, encoding)
        doc = process_css(doc)
        return doc
    end

    # Replace our tokens with html p tags. They will be left mostly untouched by Gollum's render methods
    def extract_css data
        # This method extracts all the {.css-class} lines and replaces them with a <p></p> marker
        #  so the process_css method can fix them. We do this to survive thet markdown formatter.
        # Each marker has two classes, one to uniquely identify it and another to identify them as a group.
        # this latter marker is used during clean-up.
        data.gsub /^[ \t]*\{\.(-?[_a-zA-Z]+[_a-zA-Z0-9-]*)}\r?$/m do
            class_name = $1
            id = "css-" + Digest::SHA1.hexdigest(class_name)
            @cssmap[id] = class_name
            "<p class='__marker #{id}'>save me</p>"
        end
    end

    # Find our p tags and perform the appropriate fixes
    def process_css doc
      # Re-parse the document html and then query using css selectors to find our markers.
      # Replacing the markers with css classes on the next sibling element.
      return doc if doc.nil? || @cssmap.size.zero?

      # This is dumb, render returns the html as a string, so we have to re-parse it
      # to be able to use Nokogiri methods on it.
      doc = Nokogiri::HTML::DocumentFragment.parse(doc)
      doc.css("p").each do |element|
        if element.content() == ""
          element.remove()
        end
      end

      @cssmap.each do |id, class_name|
        doc.css("p.#{id} + *").each do |element|
          element_class = element['class'] || ""
          element_class += " #{class_name}"
          element['class'] = element_class
        end
      end

      doc.css("p.__marker").each do |e| e.remove end

      # Return the xml.
      doc.to_xml(@to_xml)
    end
end

# Now begin Gollum app initialization...