Steps to configure the Google custom connection

1. Visit https://console.cloud.google.com/.
   
2. Click the dropdown and choose the required project.
   
3. Navigate to the hamburger menu on the left and click APIs and services -> Library.
   
4. You'll be taken to the API library page. Navigate to the Google Workspace section and click Gmail API.
   
5. Next, click Enable on the Gmail API product details page.
   
6. In order to use this API, you'll need to create credentials. Click CREATE CREDENTIALS.
   
7. On the Create credentials page, choose Gmail API in the Select an API dropdown and User Data under the What data will you be accessing? section. Click Next.
   
8. Next, on the OAuth consent screen, enter the App name, User support email, and Developer contact email address. Then click Save and Continue.
   
9. Next, under the Scopes section, click the ADD OR REMOVE SCOPES button.
   
10. In the Update selected scopes pane, search and select the gmail.readonly scope. Click Update, and then click Save and continue.
    
    
    
11. Now, under the OAuth Client ID section, choose Web application in the Application type dropdown. Then name your app.
    
    
    
12. Enter https://deluge.zoho.com/delugeauth/callback as the Authorized redirect URI.
    
13. Click Create and download your credentials. This Client ID and others will be required later, while creating a connection in Zoho CRM.
    
14. Navigate to the OAuth consent screen and click Add users under Test users. Enter the user's email address and click Save.
    

1. Now the action shifts to Zoho CRM. Navigate to crm.zoho.com -> Settings -> Connections.
   
2. Choose Custom Services and click Create Service.
   
3. On the Create Service screen, choose OAuth 2.0 as the Authentication type and Header as the Parameter.
   
4. Enter the Client ID, Client Secret, and Authorize URL generated from Google as shown in step 12 above.
   Tip: Having the Offline access type will allow your application to refresh tokens even when the user is not actively using the application.
5. Then enter https://oauth2.googleapis.com/token as the Access token URL and Refresh token URL.
   
6. Specify the necessary scopes like read and create, etc. and then click Create Service.
   
7. On the next page, click Create Connection. Give the connection name and choose the scopes. Then click Create and Connect.
   
8. Finally, authenticate the connection on the Google accounts page. Your custom connection will be created, and the two services can be integrated!
   

userId = "emailaddress@gmail.com";
get_gmail = invokeurl
[
url: "https://gmail.googleapis.com/gmail/v1/users/"+userId+"/messages"
type: GET
connection : "connectionname"
];
messageID = get_gmail.get("messages").get(0).get("id");

get_gmail_withID = invokeurl
[
url: "https://gmail.googleapis.com/gmail/v1/users/"+userId+"/messages/"+messageID
type: GET
connection : "connectionname"
];

Content_of_the_mail = get_gmail_withID.get("snippet");
info "Mail Content : " + Content_of_the_mail;

gmail_values = get_gmail_withID.get("payload").get("headers");
for each headers in gmail_values
{
if ( headers.get("name") == "Subject" )
{
info "Subject of the Mail : " + headers.get("value");
}
if ( headers.get("name") == "From" )
{
info "From Address : " + headers.get("value");
}
}

References