I know this is tangential, but it has come up, and you asked. The Guanzi is, as Jerry remarks, a compendium of various diverse texts. The texts titled Neiye (Inner Training) and Xinshu (Techniques of the Heart/Mind) share certain characteristics that reveal them to be what might best be term “early Daoist” in content. Some modern scholars like Harold Roth identify certain psycho-physiological practices (daoshu—techniques of the way) as the distinctive features that qualify these early documents as “early Daoist.” There has long been ambiguity in the term Daoist for a number of reasons, including a somewhat artificial distinction between daojia (school of dao) and daojiao (religion or teaching of dao). You may find interesting the essay by Nathan Sivin, “On the Term ‘Taoism’ as a Source of Perplexity,” linked below:
Sivin’s essay is already “old scholarship,” but I still think he states the issues lucidly.