How was Gondor founded?

[The Mouth of Sauron]

Pelagir was the chief haven of the Faithful before the fall of Numenor. It seems that during the Second Age the Numenorean settlements in what was to become Gondor were more peaceful, with less conflict with the original inhabitants, than was the case further north in Eriador.

For example, Minas Tirith, Osgiliath and Minas Ithil must have been at least in some form of development before the fall of Numenor - otherwise it seems doubtful whether the Faithful would have had time to build them after the Downfall and before Sauron returned to Middle-Earth.