To setup outgoing email using Gmail navigate to Settings > My Details > Setup Incoming and Outgoing Email > Gmail, then click Sign in with Google and add the email address and password you want to send emails from.
This email address doesn't have to be the same one you use to login, and more than one staff user can send emails using the same email address.
To set up incoming email you will then need to login to your Gmail or G Suite account and set up a catch all forwarding rule to send a copy of all emails you receive to your AccountancyManager account. You will find you forwarding address by navigating to Settings > My Details > Setup Incoming and Outgoing Email > Incoming Email. When setting up this rule please ensure you tick the option to keep a copy of the email in your inbox.
Once this forwarding rule is set up Gmail will send a validation email to your AM forwarding address with a link and a code. This should be validated for you automatically but if you need access to this email you can find it by navigating to Tools > Email Manager.