example of how easy gravatar support is without a plugin/gem

example_use_with_comment.rb

class Comment
  include MongoMapper::EmbeddedDocument
  include Gravatarable
  
  key :name,  String
  key :email, String
  key :url,   String
  key :body,  String
  key :created_at, Time
  
  def post
    _root_document
  end
  
  def to_liquid
    CommentDrop.new(self)
  end
end

gravatarable.rb

require 'digest/md5'

module Gravatarable
  # override this in your class if email key is not named email
  def gravatar_email
    email.downcase
  end
  
  def gravatar_id
    Digest::MD5.hexdigest(gravatar_email)
  end
  
  def gravatar(size=50, default=nil)
    url = "http://www.gravatar.com/avatar/#{gravatar_id}?s=#{size}"
    url += "&d=#{default}" if default
    url
  end

  def gravatarable?
    true
  end
end