Hi Partner,
1. When set this option to Do not allow, no email results will be stored for this contact, it's associated with email interaction.
2. If contact consent his/her name and email address:
There is an existed field called Email(it's logical name is donotemail, you can find it in form editor) with two Allow/DoNotAllow options in CONTACT PREFERENCES section;
for name, you could add a same two option custom field as Email field,
then add these fields to your subscription center page to let your contacts manage their preferences.

From operations above you filter these consent level contacts into a segment based on these field,
and you could let them join in next customer journey.
Here is a detailed consent level list for reference. while GDPR features in Dynamics 365 for Marketing is also described in the article.
Regards,
Clofly