I just tried, and no problems with sending/receiving gmail attachments at all. Everything works perfect. I did try with .doc and .pdf files, and doesn't matter what file size is the file I'm trying to send, every time works without a problem.
As you may have already noticed, avast! even doesn't check your gmail account by default. Gmail uses SSL (Secure Socket Layer protocol) and avast! doesn't support it. So, I don't see how avast can stop your attachments from being delivered (sent/received).
Cheers !