I *think* you can still board the airship. Go to the Ruby Road Exchange and go as far in as you can - past the Moogle. There should be a lift operator on the right who will take you to the airship landing. Go down there and talk to the ticketer.

If you did the quest part you need in Limsa and just need to get to Gridania, you can actually run there through Eastern Thanalan - keep running east, then north. Just don't try to fight anything past Highbridge or in South Shroud.