You could try to open up Mcafee.Click navigation,then Firewall then there is a button to restore defaults.Try that then try to access Gmail.
Can you please confirm if you are able to access Gmail with McAfee firewall turned OFF ?
Please follow the instructions here to disable McAfee Firewall temporarily and check the status ;
- Double-click the M icon in your taskbar.
- Click Navigation.
- Click Firewall.
- Click Turn Off.
NOTE: The McAfee icon will turn gray, and display a red and white exclamation symbol. Additionally, the color bar in SecurityCenter will turn red and report your computer is at risk. Both of these issues will resolve when you turn the firewall back on by clicking the Turn On button.