Obviously your email address has been harvested, have a gmail address that I very rarely use it still manages to get spam, but not much and Gmail does a good job of sending them directly to my Spam folder. Only recently have I had one arrive in my inbox that had to be dealt with manually.
I don't know if you have changed your Gmail settings, but this really is something you need to address with Gmail and not really in the Avast Forums.
You could try a google search 'how to block multiple spam messages in gmail' - this is what I got -
https://www.google.co.uk/search?q=how+to+block+multiple+spam+messages+in+gmailYou might even consider changing your Gmail email address if it is getting this kind of volume I would certainly be considering that if it were me.
