Packet Broker currently routes traffic from gateways connected to The Things Network V2 to V3. That is, gateway coverage of V2 is available in V3. But not the other way around : gateways already connected directly to V3 will not be available for applications in V2.
As a friendly request, both as a user of TTN and as the maintainer of TTN Mapper, please keep your gateways on TTNv2, until as late as possible.
People in your city that have not yet migrated their application from v2 to v3 will lose connectivity if you migrate your gateway.
TTN Mapper does not currently map gateways connected via the Packet Broker. This is a work in progress. To keep your coverage on the map, please leave your gateways registered on V2 for now.