Google misrepresented the ad as the product of the Gimp project, and were paid as a result. They usually use an "obeying the law would not scale" type argument in court, but that would clearly be bullshit in this case. They have a business relationship with the ad buyer, and should have verified their affiliation with gimp.org. Also, a simple string match on the URL would expose the attempted fraud on Google's end.
I'm not sure how to check if Gimp is a registered trademark in the US. This page kind of implies it might be (or that the author of the page does not understand trademarks):