So you are saying if a German citizen travels to Mexico, they don't need to obey Mexican law because they are German citizens?
Suppose a law in Mexico requires Google to store search queries and make them available to the government for 1 year, or it requires the collection of certain types of personal information, and a law in Germany bans Google from storing search queries or collecting the information. Now a German citizen travels to Mexico, and does a web query in Mexico, do you think Google should apply Germany's Laws or Mexico's laws? It will apply Mexico's laws because that is the jurisdiction in which the query is made. Similarly if a Mexican travels to Germany and makes a query, then Google will not store the results.
Btw, this is the whole point of international VPNs. People want an internet presence in different countries in order to access content that is not available in their own country or to be treated differently than if they were in their own countries. So if you, as the German traveller, don't want your query stored, you'd VPN to a server in Germany and run your queries through that VPN. If you ran your query through the Mexican ISP, you can be sure that the information would be collected.
Thus as much as European governments may want the GDPR to be a type of shield that you can carry with you when you cross over to other jurisdictions, the reality of that portability is limited to the ability of European nations to convince other nations to go along and treat Europeans differently in their own legal system. It may work, it may not, but whether it works is not a question of the GDPR but of the ability of Europe to project power and override laws in other jurisdictions.