Any plug-in would be for Outlook Express, the same as for MS Outlook/Exchange, so you would run the plug-in rather than the Internet mail provider, so effectively avast is working inside the email program not outside.
This means there isn't a specific requirement to be able to handle Gmail, rather to be able to integrate with OE.
However, there is no way it will be in 4.7, that would be a large project not just a minor update. I haven't heard anything about the intentions of Alwil to create an OE plug-in, so it may well be a long wait.