Starting from Postcall
In Postcall, start by going to your account's setup page.
There, you will be able to enter your Gorgias account subdomain, and then start the process of connecting your account:
Starting from Gorgias
In Gorgias, you can find the Postcall App Store listing here: https://www.gorgias.com/apps/postcall.
From there, you can initiate the installation of the integration.