I have found fork of one gem in github, which is more up to date. I wanted to install this fork to my rails project. So I have added this line of code to my
gem 'rootapp-wkhtmltopdf-binary', github: 'Root-App/wkhtmltopdf_binary_gem'
bundler is not actually hanging, it is downloading the content of the repository in the background.
The repo-content seems to be of a significant size (multiple megabyte). Depending on your connection you will have to wait a while for bundler to finish downloading. Afterwards it should install the gem as usual.
Unfortunately bundler is not giving any feedback on this, even when you invoke bundler in verbose mode
bundle install --verbose