You probably know that when you try to receive SpamGourmet emails they never come through if you use Proton Mail. So, we're going to use another masking service (it can be a good one with very ungenerous features, we just need one mask). In this tutorial I use Firefox Relay, but you can use anything else (like AnonAddy, but not SimpleLogin, as it probably won't work.)
Without further ado, let's go.
1. Sign up for Firefox Relay <https://relay.firefox.com/accounts/fxa/login/?process=login> (use your real email)
2. When logged in, click "Generate new mask".
3. Add an account name if you like.
4. Copy the email address
5. Now, if you already have a SpamGourmet account, simply change your protected address to the Firefox Relay one, and verify it.
6. If not, simply create a new SG account with the FF Relay email, and verify it.
Note that there are 2 caveats: 1) you can't send replies (if SG can even do that) 2) you will get a large banner at the top of your messages advertising Firefox Relay's paid plans like this: