Salem

I found an abandoned place, and I claimed it. But is it possible to replace the locks of the gates which are on my claim, by applying a new lock on it?

If it already has a lock on it, then no. Only option is for you to bash the front gate and then rebuild it.

If you are holding a master key for the gate then you will be able to apply a new lock, provided that the master key is attuned to your character.

Oh, I see. Thank you.

Also, is there a way to get more granite boulders? Or do you really have to forage for granite if you're out of the boulders?

Granite oulders only spawn when the map is first generated, the same goes for wood heaps.

Ah, thank you.