When sending mail merge campaigns in Gmail with GMass, you can already suppress a list of addresses that belong to a prior campaign. Here is a detailed guide on using Suppression Lists in GMass. Additionally, if you're using the GMass unsubscribe link, then GMass is already suppressing anyone on your Unsubscribe List.

You may, however, want to use a Google Sheets spreadsheet as the source of a suppression list, such that you can add new email addresses to just the spreadsheet, and have all of your future mail merge campaigns automatically suppress any email address that is on the spreadsheet.

To accomplish this, you have to set up a spreadsheet to hold the email addresses, and then create a container email campaign that will continuously update itself with the email addresses in the spreadsheet. You can do this with a combination of our mail merge and Google Sheets integration, the recurring campaigns feature, and the option to "Just create Drafts".