To install the app you will need to be an administrator of your Jira instance.
Visit the marketplace listing - https://marketplace.atlassian.com/apps/1219538/google-ical-calendar-dashboard-gadget-for-jira and click the “Try it free“ button
Wait for the installation to finish.
This adds the app to your Jira instance and you are ready to use it. Next, read Finding Google & iCal Calendar Dashboard Gadget on where to find the app in Jira.