Google filters viruses sent or received through Gmail by using a heavy handed approach to filtering. Rather than scanning each email for viruses, they simply prevent people from sending or receiving files with .exe extensions, even if they are in a zipped folder.
Getting around this is very simple, but you must let the recipient know what's going on.
Anytime you want to send a .zip file rename the extension. I like to just take the "p" off of the end. For example, I will name the file – file.zi. In the body of the message I tell the recipient to add the "p" back on before trying to open the file.
