ECG and blood pressure aren't available in the UK yet - it's 'coming soon', I guess it has to get regulatory approval from some form of health/drug body like it has to in the States.
Gmail - There is no 'Gmail' app per say, the Watch will show you notifications for new emails as they come in as notifications on your phone. In the Galaxy Wearable app, go to Notifications and ensure it's turned on for Gmail. As you get an email, the watch will show the notification which you can click and read, or you can delete the email from the watch or just dismiss the notification by swiping up on the notification on the watch.
When I say you can read, I think you only get a simplified text only version and not the full blown email with HTML formatting etc. As there's no Gmail app, you can't go looking at your historical emails.
There's an Outlook client on my Galaxy Watch 3, but I've not used it. Whether or not you can set up access to Gmail in Outlook on your phone, which the watch app can then access as a full blown email client with a list of all emails, I don't know. You'll have to give that a try.
