Comment by yllus

Sorry, use of the term GUID confused things - I meant that if an identifier string is generated when you click on the ad, and the purpose is to simply see if that identifier completes the app install and first use - that's not against GPDR. (In my head GUID means "unique identifier string".) Storing the GUID tells you nothing other than some device clicked on an ad and some device did or did not complete the app install.