`ERROR: Could not find a valid gem 'jekyll' (>= 0), here is why: Unable to download data from https://rubygems.org/ - bad response Gateway Time-out 504 (https://tokyo-m.rubygems.org/quick/Marshal.4.8/jekyll-2.0.1....
Is there any way to specify other mirrors, and is there other mirrors than the Tokyo one that would work better?
Assuming you are using Bundler, and not trying to install the jekyll gem globally, you can manually specify a server in the Gemfile.
In an empty directory, I created a Gemfile with only these contents:
source 'http://production.s3.rubygems.org'
gem 'jekyll', '~> 2.0'
[mason@IT-PC-MACPRO-002 jekyll-2-fuckery]$ bundle install --path vendor/bundle
Fetching source index from http://production.s3.rubygems.org/
Resolving dependencies.................
Using blankslate 2.1.2.4
<bunch of lines deleted...>
Your bundle is complete!
It was installed into ./vendor/bundle
[mason@IT-PC-MACPRO-002 jekyll-2-fuckery]$ bundle exec jekyll --version
jekyll 2.0.2
[mason@IT-PC-MACPRO-002 jekyll-2-fuckery]$
(EDIT: Updated with server that worked.)