It sounds like you need to turn on either the "Incoming Uses SSL" option, or the "Outgoing Uses SSL" option, within the Advanced section of the Mail Settings for your G-Mail Account.
Settings -> Mail -> G-Mail Account -> Advanced, and make sure both options are set to On.
If you already have these options setup correctly, then you'll need to provide some more details (like are you on your own or a public network, behind a firewall, etc.)