Yes since GDPR states that personal data is any piece of information that uniquely identifies a living person, creating a unique identifier for each visitor by definition will make you a data processor.
IP addresses are slightly different because that address can be used to identify the subscriber in certain cases (who in turn may or may not be an individual).
Obfuscation of the ID doesn't help.
If you or a data processor you use, uses the data for secondary purposes not covered by any exemption to opt-in consent, I believe you would have to get opt-in consent for those secondary purposes beforehand.
Note: the cookie law is the ePrivacy directive (and national interpretations like PECR) and it goes beyond GDPR in some ways, as the ICO states "Although cookies that process personal data give rise to greater privacy and security risks than those that process anonymous data, PECR apply to all cookies." ( https://ico.org.uk/for-organisations/guide-to-pecr/cookies-a... )