All users have identities on many popular, centralized online platforms.
With many of these services, it's possible to log into other services with a primary service.

PGP has a totally different approach, you have to actually meet, and then verify someone's identity
and then you can sign their key. Did I mention that not many people use PGP.

However, really, in the online world it's these centralized services that are the arbiters of identity.

so: here is how you could bootstrap decentralized identity of these centralized services:

Sign a claim that you are a given identity, and then upload it to a place where only that centralized identity can access.
like https://github.com/USERNAME/pubkey if you do not have control over the URL (like on twitter, you don't know the id for a tweet until twitter decides) then you'll have to create another object that points to url for the claim and sign it too.

Then other users can automatically check your identity via the URL and sign it if they verified it.