IANAL but as I understand GDPR, this is incorrect. The paragraph you cite discusses personal data. Google's FAQ on GA is instructive (emphasis mine) [0]:
> When using Google Analytics Advertising Features, you must also comply with the European Union User Consent Policy.
They admittedly keep things as vague as they can, but to me it kind of reads like: using GA to collect site usage analytics is actually fine and requires no explicit consent as long as you've configured it to anonymize the IP addresses (toggle this in GA) and you're not tracking e.g. user IDs and such.
Similarly, using GTM to deliver a paragraph like OP did is also fine.
In both cases the spirit and the letter of the law would seem to be respected if you add some notice about tracking going on in your footer. No explicit consent is needed here, because no personal data is getting tracked.
Edit: clarity.