Add a source and remove the default source
$ gem sources --add https://mirrors.tuna.tsinghua.edu.cn/rubygems/ --remove https://rubygems.org/
$ gem sources -l
Or edit ~/.gemrc, add the following to sources section:
https://mirrors.tuna.tsinghua.edu.cn/rubygems/
see https://mirrors.tuna.tsinghua.edu.cn/help/rubygems/
Add mirror for bundle
$ bundle config mirror.https://rubygems.org https://gems.ruby-china.com
$ bundle config mirror.https://gems.ruby-china.org https://gems.ruby-china.com