Anyone who has concerns can read the Gmail privacy policy here. This is in NO WAY spyware. Period. Here's an excerpt from the privacy policy:

"We never share your personally identifying information with any advertiser as part of this service, unless you specifically ask us to do so. When you are logged into your Gmail account, Google will display targeted ads and other relevant information based on the content of the email displayed. In a completely automated process, computers process the text in a message and match it to ads or related information in Google's extensive database. No human reads your mail to target ads or other information without your consent.

Advertisers receive a record of the total number of impressions and clicks for each ad. They do not receive any personal information about the person who viewed the ad. If you click on an ad, Google will send a referring URL to the advertiser's site identifying that you are visiting from Gmail. Google does not send personally identifying information to advertisers with the referring URL. Once you are on the advertiser's site, however, the advertiser may collect personal information about you. Google does not control or take responsibility for the privacy policies of other sites."