Will my white label portal bring me customers from Google on its own, or do I still have to do my own marketing?

No, a white label portal will not bring you traffic by itself, and it is only fair to say that plainly. The portal gives people a good place to search and book once they arrive, but getting them to arrive is still your job, in the same way a shop window does not put people on the street outside.

The reason is that search and results pages are built the moment a customer asks for them, from live fares and availability, so there is nothing fixed for a search engine to store and rank. Pages that do hold steady content, such as your home page, a destination page or questions and answers you write yourself, can be found in search over time, and those are the ones worth your effort. How much content you can add, and how much control you get over it, depends on your plan and on how your site is set up.

The portal works best as the place people land rather than the place they discover you. Most agencies get their bookings from customers they already have, from social posts and messages, from ads pointing at a specific trip, and from being the agency people already call. Any of those can send someone straight into a search on your portal with the trip details already filled in for them.

Before you launch, decide where your first visitors will come from, because a portal with nobody pointed at it will stay quiet no matter how good it looks.