Custom Ruby Gem in Gemfile on Heroku

You can place custom gems in the vendor directory and then specify them in your Gemfile:

gem 'gemname', '1.0', :path => 'vendor/gemname'

Unable to push a custom gem to heroku master

When Heroku "reads" /home/nci/Zacchi/kuizu/gemz (or any other local reference) it doesn't know where to go. You need to publish your gem and remove the local reference from your Gemfile.

How to install gems manually in heroku?

You can not manually maintain application dependencies via Heroku CLI.

When you deploy to Heroku, assuming you have picked 'Ruby' or 'Rails' as the Heroku application type, the buildpack it will bundle for you. https://devcenter.heroku.com/articles/getting-started-with-ruby#deploy-the-app
You can find your app type and build pack here:
https://dashboard.heroku.com/apps/HEROKU_APP_NAME/settings

remote: -----> Ruby app detected
remote: -----> Compiling Ruby/Rails
remote: -----> Using Ruby version: ruby-2.3.4
remote: -----> Installing dependencies using 1.7.12
remote:        Running: bundle install --without development:test --path

You should also know that Heroku Dynos are ephemeral. You do not have a dedicated server where you can SSH in and make file / permission changes that persist. Anytime your dyno resets or you push new code to production, your entire file system is 'regenerated' from git.

Custom Ruby Gem in Gemfile on Heroku