Google API Generate Access Token and Refresh Token

Steps :

a) Generate client id and secret key.

b) Add Google logged in user as a Test user in consent screen.

c) Generate Authorization Code (One Time)

https://accounts.google.com/o/oauth2/auth?client_id=xxxxxxxx&redirect_uri=http://localhost&response_type=code&scope=https://www.googleapis.com/auth/drive&access_type=offline

d) Generate Refresh Token (One Time)

curl --request POST --data "code=xxxxxxxx&client_id=xxxxxxxxxxxx&client_secret=xxxxxxxxxxxx&redirect_uri=http://localhost&grant_type=authorization_code" https://oauth2.googleapis.com/token

e) Generate Access Token from Refresh token (Always)

curl --request POST --data "client_id=xxxxxxxxxxx&client_secret=xxxxxxxxxxxxxxx&refresh_token=xxxxxxxxxxxxx&grant_type=refresh_token" https://oauth2.googleapis.com/token

Note : Refresh Tokens expire in 1 week if your app is not set as production. Change publishing status of your app from testing to production to use your refresh token always.

The Publishing Status option can be found in 'Oauth Consent Screen' which is under API & Services.