You can assume that the companies selling off email data are smart enough to do the entirely trivial "remove + sign to @ sign" transformation for gmail addresses, at least partly because their job tends to be tracking you across a large amount of domains.

I switched to mails under domain I own (and powered by FastMail) some time ago; I now use [email protected] form. Try to filter for that without breaking non-aliased e-mails!

By that point you might as well set a fixed-width length and treat everything after that as an alias, like [email protected] would be the base and [email protected] would be your alias for spammers.com, etc. Even better, put the alias before the username and keep the + as a separator.

Come to think of it, I bet doing this actually gives them better signals than they'd otherwise get, because if they receive emails by word of mouth, then they get additional context as to what sites you're signing up to.