AppiWorks for Gmail is available in Google Workspace Marketplace
You can install AppiWorks in 2 ways.
Open Gmail.
In the add-on section, click on the plus icon.
Search for "AppiWorks".
Select AppiWorks and install the add-on.
It will take you to the Authentication window and select your account.
click Allow that will complete your Authentication process.
That's It. your installation process is Completed.
Directly reach the add-on using the following link in Google Workspace Marketplace AppiWorks for Gmail at Google Workspace Marketplace
Click on Install.
Follow the steps above.