Sub-addressing of the format username+foo@host pre-dates Gmail. I think what is unique to them is being able to include periods anywhere within the username, e.g. us.ern.ame@host, user..nam.e@host, username@host are all equivalent.
which is useful for testing delivery to gmail, but ideally you want to have a really mixed bag of receiving smtp daemons with different spam/abuse filtering profiles (all of which are pretty much opaque these days for anti spam reasons) and different services to test reliable delivery.
https://en.wikipedia.org/wiki/Email_address#Sub-addressing