Thank you all. I followed the guide and im facing this error : 
The same error i get in SMTP settings.
Im using the same client ID and same client secret for OSticket itself and for that 1 email that is going to be as a support email (which is the main one).
I was able to pass the google authentication and I selected "Allow" when google asked for premission.
Note 2: Osticket v1.17
Hosted: Localhost (internal only via IIS)
Please help :/