Gem 'pg' not installed when migrate from mysql to postgresql


I tried to migrate my gitlab 8.17.4 from mysql to postgresql following this guide, but gem Pg is missing.

When i run command below:

$ bundle install --without development test mysql --deployment

Don’t understand why gems in postgres group are not installed!

Bundle complete! 187 Gemfile dependencies, 265 gems now installed.
Gems in the groups development, test, mysql and postgres were not installed.

Any idea?