In order to integrate Work Time Calendar with google calendars you need to enable Google Calendar API and register Google Client Id. Below are the instructions with screen-shots.
Please navigate to https://console.developers.google.com and login under google account that is associated with your company. If you do not have company google account, just create a new one.
Click Create Project, type some name and press Create.
In newly created project go to Library -> Google Apps APIs -> Calendar API and press Enable API button.
Click "Go to Credentials" button at the right and fill the options as shown on the picture.
Next you will need to put "Email address" and "Product name" for the consent screen. This step is omitted if you configured the consent before.
Finally you are presented the client ID. Please copy it. Press Done to finish the Calendar API configuration.
Once created, please go back to JIRA and open Jira Administration -> Add-ons -> Work Time Calendar section -> Google Integration.
Tick the Enable checkbox. Paste the client ID to Google Client ID field and press Save. Please make sure to put correct client ID since wrong value can break functionality of Work Time Calendar for all users.
You can leave the calendar table empty so users configure their own calendars. Other option is to set calendars that are common for your company or all JIRA users. Here you can add up to 4 calendars. Good candidates for common calendars are state holidays and your company calendars. You can find list of all public calendars by opening your google calendar, clicking the drop-down menu near "Other calendars" and selecting "Browse Interesting Calendars".
In addition to company-wide calendars users can add up to 4 their own calendars. This gives a total of 8 calendars per user.
When configuring your company calendar, make sure it's visible under google accounts of your JIRA users. If there are no security restrictions, the simplest way is to make the calendar public.
Please note that google calendar API has free quota of 1,000,000 requests per day and you need to pay when it's exceeded and you still want to have access to the API. This can happen only for huge JIRA instances where thousands users actively use Work Time Calendar.