"free use of Gmail" but only if you're using Google's email client. If you're using the POP (and, presumably, IMAP) API, you're not going to see any ads, amiright?
When using any POP/IMAP client, you won't get ads as the POP/IMAP protocol has no provisioning for it. Unless Google injects the ad into the email body (which is near impossible to do reliably) they cannot show ads even if they want to.