I revised to prevent from occurring exception which is caused by nil. If you put gist without language field, exception occurred. Then I put nil check.

gist_tag.rb

# A Liquid tag for Jekyll sites that allows embedding Gists and showing code for non-JavaScript enabled browsers and readers.
# by: Brandon Tilly
# Source URL: https://gist.github.com/1027674
# Post http://brandontilley.com/2011/01/31/gist-tag-for-jekyll.html
#
# Example usage: {% gist 1027674 gist_tag.rb %} //embeds a gist for this plugin

require 'cgi'
require 'digest/md5'
require 'net/https'
require 'uri'
require 'json'

require './plugins/pygments_code'
require './plugins/raw'
require './plugins/octopress_filters'

module Jekyll
  class GistTag < Liquid::Tag
    include HighlightCode
    include TemplateWrapper
    include OctopressFilters

    def initialize(tag_name, text, token)
      @title = nil
      @caption = nil
      @highlight = true
      super
      @text           = text
      @cache_disabled = false
      @cache_folder   = File.expand_path "../.gist-cache", File.dirname(__FILE__)
      FileUtils.mkdir_p @cache_folder
    end

    def render(context)
      if parts = @text.match(/([\d]*) (.*)/)
        output = super
        gist, file = parts[1].strip, parts[2].strip
        script_url = script_url_for gist, file
        json = get_cached_gist(gist, file) || get_gist_from_web(gist, file)
        files = json["files"]
        url = json["html_url"]
        description = json["description"]
        source = ""
        files.each do | name, info|
          if !file.empty? && file != name
            next
          end
          code = info["content"]
          lang = info["language"].nil? ? nil : info["language"].downcase
          if lang == "markdown"
# マークダウンのレンダリング
#            source += pre_filter(code)
#            partial = Liquid::Template.parse(source)
#            context.stack do
#              partial.render(context)
#            end
          else
            caption = "<figcaption><span>#{description}</span><a target='_blank' href='#{url}'>link</a></figcaption>"
            code = "<figure class='code'>#{caption}#{highlight(code, lang)}</figure>"
            code = safe_wrap(code)
            code = context['pygments_prefix'] + code if context['pygments_prefix']
            code = code + context['pygments_suffix'] if context['pygments_suffix']
            source += code
          end
        end
        source
      else
        ""
      end
    end

    def script_url_for(gist_id, filename)
      url = "https://gist.github.com/#{gist_id}.js"
      url = "#{url}?file=#{filename}" unless filename.nil? or filename.empty?
      url
    end

    def get_gist_url_for(gist,  file)
      "https://api.github.com/gists/#{gist}"
    end

    def cache(gist, file, json)
      cache_file = get_cache_file_for gist, file
      File.open(cache_file, "w") do |io|
        JSON.dump(json, io)
      end
    end

    def get_cached_gist(gist, file)
      return nil if @cache_disabled
      cache_file = get_cache_file_for gist, file
      #data = File.read cache_file if File.exist? cache_file
      data = nil
      if File.exist? cache_file
        open(cache_file) do |io|
          data = JSON.load(io)
        end
      end
      data
    end

    def get_cache_file_for(gist, file)
      bad_chars = /[^a-zA-Z0-9\-_.]/
      gist      = gist.gsub bad_chars, ''
      file      = file.gsub bad_chars, ''
      md5       = Digest::MD5.hexdigest "#{gist}-#{file}"
      File.join @cache_folder, "#{gist}-#{file}-#{md5}.cache"
    end

    def http_get_data(uri)
      raw_uri           = URI.parse uri
      proxy             = ENV['http_proxy']
      if proxy
        proxy_uri       = URI.parse(proxy)
        https           = Net::HTTP::Proxy(proxy_uri.host, proxy_uri.port).new raw_uri.host, raw_uri.port
      else
        https           = Net::HTTP.new raw_uri.host, raw_uri.port
      end
      https.use_ssl     = true
      https.verify_mode = OpenSSL::SSL::VERIFY_NONE
      request           = Net::HTTP::Get.new raw_uri.request_uri
      data              = https.request request
      if data.code.to_i != 200
        raise RuntimeError, "Gist replied with #{data.code} for #{uri}"
      end
      data
    end

    def get_gist_from_web(gist, file)
      gist_url          = get_gist_url_for gist, file
      data = http_get_data(gist_url)
      json = JSON.parse(data.body)
      cache gist, file, json unless @cache_disabled
      json
    end
  end

  class GistTagNoCache < GistTag
    def initialize(tag_name, text, token)
      super
      @cache_disabled = true
    end
  end
end

Liquid::Template.register_tag('gist', Jekyll::GistTag)
Liquid::Template.register_tag('gistnocache', Jekyll::GistTagNoCache)