Thran Portal will have the appropriate intrinsic mana ability for the basic land type chosen as it enters the battlefield. It is still a Gate and still has its other abilities, including the last ability, which makes the mana ability associated with its basic land type cost 1 life to activate.

You still have to pay that mana ability's other costs. Usually, this is just tapping the land.

If Thran Portal somehow gains another mana ability, that ability also costs an additional 1 life to activate.

If you control more than one Thran Portal, the last ability of each of them applies only to itself. That is, controlling a second Thran Portal does not make mana abilities of all cards named Thran Portal cost an additional 2 life.

If a Thran Portal somehow enters the battlefield without a basic land type having been chosen for it, it does not have any mana abilities. The same is true for a land that is already on the battlefield that becomes a copy of it.

If a land enters the battlefield as a copy of Thran Portal (as opposed to a land that is already on the battlefield becoming a copy of it), its controller gets to choose a basic land type for it and it has that type.