What do you mean by this? A users information on a standard Blocs site is not mandatory. The only issue you face is the IP data being transferred via to 3rd parties that would be in breach of GDPR. But like I’ve mentioned before Google Analytics offer a work around to make the IP less accurate and Blocs now has the ability to use local web fonts so you no longer need to use Google fonts exclusively in Blocs sites.
Now with those two issues addressed what personally identifying data will a Blocs site store that would break the GDPR terms? Maybe someone contacts you via email, so you get their contact information via your site. Well you can now include a checkbox (2.5.3) on your contact forms that is required to be checked in order for the form to be send, you can explicitly state how you will handle their data when they sent it to you before they do.
Adding a popover to opt in on an entire site is not acceptable according to the GDPR rule book, it would need to state every way you plan on using the visitors information and have a checkbox for each item. But since Blocs creates static web sites without features like logins etc, you are literally creating a popover that A. does not meet the requirements of the GDPR and B. is pointless because your site is not storing personally identifying information unless its submitted by the user if you adjust analytic’s and don’t use Google fonts.
To be honest I recommend you read up on GDPR as I’m not sure you fully understand how to comply with it. A simple popover on a page asking you to opt in is not the correct approach and would give you zero cover.