The GDPR roughly applies in the following scenarios:
- Art 3(1): you have an establishment in Europe
- Art 3(2): you do not have an european establishment, but
- Art 3(2)(a): offer goods or services to persons in Europe
- Art 3(2)(b): monitor the behaviour of people who are in Europe
(where Europe means EU/EEA/UK as appropriate).
Art 3(1) does not seem to apply for you. Art 3(2)(a) does not apply, since you're not actively targeting people in Europe. At the point in time where you are offering the app to users, those users are in the US.
Art 3(2)(b) could apply if you collect some kind of tracking data, in particular (but not limited to) location data. But if you temporarily shut down collection of new data for personalisation while the user is in Europe, that's probably going to be reasonably safe. It might not be necessary to disable ad personalisation if that personalisation is based on data collected outside of Europe. In practice, unless your app is specifically targeted at travellers, no one will care about what your app does outside of the US.
For detailed guidelines on the territorial scope of the GDPR, consider reading EDPB guidelines 3/2018 (PDF). The document contains some relevant examples, but since it's official guidance they won't explicitly say that GDPR won't apply in a scenario like yours. The closest is Example 8:
An Australian company offers a mobile news and video content service, based on users’ preferences and interest. Users can receive daily or weekly updates. The service is offered exclusively to users located in Australia, who must provide an Australian phone number when subscribing.
An Australian subscriber of the service travels to Germany on holiday and continues using the service.
Although the Australian subscriber will be using the service while in the EU, the service is not ‘targeting’ individuals in the Union, but targets only individuals in Australia, and so the processing of personal data by the Australian company does not fall within the scope of the GDPR.
Also relevant is Example 10, which says that app downloads in the EU might not be subject to GDPR:
A U.S. citizen is travelling through Europe during his holidays. While in Europe, he downloads and uses a news app that is offered by a U.S. company. The app is exclusively directed at