If your MTA supports a catch all, why can’t you forward replies with the same feature, and drop gmail altogether?
The envelope from is the address that has to support VERP, because the only way to reliably identify the cause of a bounce is via information in the original email’s envelope from address, as Michael previously explained.
I have my doubts about that assertion. Using a gmail address for replies is janky, and always has been janky; the mail-receiver is a much more straightforward, reliable, and low-jank way to handle replies (and bounces).
Even for those small sites using gmail, the really tiny ones are probably ignoring the ToS and using gmail to relay outbound as well, and the vast majority of the rest probably aren’t beholden to an MTA that doesn’t support detail addressing.