Download all gems dependencies

I did

export GEM_HOME=.
export GEM_PATH=.
gem install --no-ri --no-rdoc --install-dir . <gem>

Thats exactly the problem I had.
After searching around a while I found a Solution who works using Bundler https://bundler.io/

Getting Gem with Dependencies:

• Create a new Folder with a File named Gemfile in it.
• Write a Source and the Gem you want to have the dependencys for into the File
• Example:

  source "http://rubygems.org"
  gem 'rails', '3.2.1'

• Open a Commandline at this Folder an Execute: bundle install
• This should download and install all Dependencys
• Execute the Command bundle list if you wanna see it
• Execute the Command bundle package
• This should create the Directory Structure vendor/cache
• Inside the cache Directory are now all the Dependencys you need for your gem

Install Gem on Machine without internet connection:

• Copy the cache Folder to the Machine
• Open a Commandline inside the Cache Folder and execute gem install --local Gemname.gem
• Example:

gem install --local rails-3.2.1.gem