HubSpot is the system we use to send emails. While Gmail is still a system we use for sending internal. logistical, and planning type emails. HubSpot is for broader communication to larger audiences.
The ability for a contact to manage their email preferences is linked at the bottom of every email sent from HubSpot
A user can only belong to 1 chapter at a time.
If a user makes a donation to your chapter it does not mean they have chosen to be on your chapter email lists, they choose to give money to your chapter. However they can update their chapter preferences at any time.
Check out the blog "Email Best Practices and Why We Need Them".