Hmmm..
As an app developer with Admob Ads, I store no personally identifiable information about the people using my app. Google's APIs take Google-stored information and pass it on to other Google services to provide personalised apps. Many of those APIs take pains to ensure that I as a middle-man cannot see or modify that data. Does GDPR therefore apply to me?
Frankly I'm unimpressed by this legislation that has (deliberately?) introduced a vast range of grey areas that are being enthusiastically exploited by rent-an-experts, consultant services and others. From photography forums (are photographs GDPR compliant?) to cake shops (are we responsible for people using our hashtag?) the 'protection of the people' introduces more confusion than solution.