A lot of people might have tried to integrate their vTigerCRM applications with their Google Calendar and have probably failed to do so. Here is the link to the official vTiger instructions:
From personal experience however I can say that the wiki page is useless and outright wrong. Here are detailed steps on how to get started with the integration. The guide below assumes that you already have vTigerCRM installed on your hosting account.
First go to the Google Developer Console here:
You need to be logged in with your Gmail account first, if not, it you just sign in. Create a new project:
The next step would be to click on the project we just created. This will lead us to the following screen:
Click on APIs:
Click on Calendar API:
Hit the Enable API button and then Credentials from the left side menu:
Click on Add Credentials -> OAuth 2.0 client ID. You will be redirected to the following screen:
Click on Configure Consent Screen:
Fille in a Product Name, which is the only mandatory field and hit Save. This will redirect you to the previous screen, but the Application types will now be active. Please select Web Application and fill in the details as shown on the screenshot below.
Just replace YOURVTIGERDOMAIN with the actual domain name where vTiger is installed on your hosting account.
Once ready, hit Create and you will be provided with your OAuth Client ID and client secret:
Please write these down and keep them for reference as we will be needing them a bit later during the setup.