avast doesn't block but scans content and alerts to any infection. So if there is any blocking going on it is usually a conflict between two or more applications and most commonly the firewall.
What is your firewall ?
What error messages are being displayed ?
As you can see from my image, that isn't a problem for me, I clicked your link and was redirected to the https secure logon page.
When you go from an http (non-secure) page to an https (secure) page some firewalls with have privacy features that can have this apparent blocking effect as it clashes with the web shield proxy.
Have you tried a direct bookmark connection to the https logon page ?
There are many forum members that also use gmail and if this were a common problem we would certainly see more on the forums.
