Interested parties will probably already be using free software to remove ads across the web, but you have a problem with this free software that removes ads only in gmail and provides a ton of other neat features? Adblock asks for donations too -- https://adblockplus.org/en/contribute.
But why hidding /removing the ads in gmail feel wrong? Because they are not too visible. The only bad/intrusive part of gmail ads is the mail parsing to select which ad will be shown. I have no proof, but I strongly suspect this will be done even if I hide them, so nothing to win there.
In regards to what we are giving in exchange for the free service, there's all sorts of tracking activity, contact and content mining, and general Google account-related user information that we share with Google in order to feed Google's database.
Google aims to feed its database to improve its products and make its products more attractive to partners. Ads in Gmail are only part of the story of how Google gets rich from your stream of personal data. A few ads shown on the Gmail interface is not how Google is making its billions.