@JLaugharne7 the gmail extension for HubSpot wouldn’t interfere at all with your ability to send emails or reach your recipients.
When you use the extension, there is a bcc address added that allows the log/track functionality to work - since you’re not using this option, it is really only acting as a shortcut to your CRM from Gmail.
The more likely cause is that you don’t have your DKIM, DMARC, and/or SPF propertly setup for Gmail/GMASS.
While those changes go into effect officially today, many “beta” features were available to users which could impact your results over the last few weeks.