First go to your Settings, in the Senders section and click New Sender.
Select your preferred Sender type: Gmail sender. Note that you can also set up a custom sender, more infos here.
Go through the Google authentification to connect your account (it can be a personal email @gmail.com or a Google workspace email). If you have an Outlook domain, please connect a custom sender.
On the Google permission page make sure to accept the permission "Send on your behalf"
And that's it, your account is connected and you're set up to start sending emails from SoftSync through your Google account.