You will click here for create the google service account 

You will visit here for create the service account and download the JSON key and .p12 key from service account portal.

Refer the below screenshot

Click on email 

Click on KEYS

Click on ADD KEYS 

Selects Key type and download JSON key and p12 key and upload in "Service Account Keys" Screen

Click EDIT

Add GetAuthToken action in your project using the reference component GoogleServiceAccount_IS

There are two types of the keys 1. JSON key and 2. P12 key.

You will provide the Key Type is JSON, component will refer the json file from database and get the google auth token.

If you will provide the P12 and JSON key ... Read the JSON file and P12 key certificate and generate the JWT token. Which will refer as assertion parameter in API integration. Grand type parameter should urn:ietf:params:oauth:grant-type:jwt-bearer   

Note : grand_type and assertion values should be url encoded then send to API.

Refer Demo Screenshot for implement the components :