How can I obtain a link (a URL) to a specific email in GMail? I want to click that link and open the specific email.
I don't know about a specific "email", but you can view a specific thread (which is usually one email) by clicking in the URL bar and copying that. Then, change the label to "all".
So if the url is "https://mail.google.com/mail/u/0/#inbox/abc123def456", you would change "#inbox" to say "#all" like this: "https://mail.google.com/mail/u/0/#all/abc123def456"
Now the link will work, even after you archive it and it's not in your inbox.