gossip model is based on NIP-65, which is where users publish an event that specifies where they publish events, and where they look for responses to their events (and inbound DMs) also known as "outbox model" which is a much more accurate name blastr is just opportunistic broadcast to everyone who will accept a new event my opinion is that opportunistic broadcast favours the biggest relay (most bandwidth and storage) whereas gossip/outbox model is more horizontally scalable and doesn't bias traffic towards central nodes