Sending personally identifiable information (PII) to Google Analytics is one of the things you should really avoid doing. For one, it's against the terms of service of the platform, but also you will most likely be in violation of national, federal, or EU legislation drafted to protect the privacy of individuals online. In this #GTMTips post, I'll show you a way to make sure that any tags you configure this solution with will not contain strings that might be construed as PII.
PII (Personally Identifiable Information) is something we need to actively combat against when using Google Analytics, as the platform explicitly forbids sending PII to Google Analytics properties in any size, form, or shape. One of the most common ways of accidentally passing PII to a property is via query parameters. Many email platforms out there, for example, see no problem in including the user's email address in the query string, especially when the user follows a link in a newsletter.