Looking like an ordinary tree

[Anthony]

Originally Posted by Rasputin

How do I model that in GURPS?

You declare that they look like trees. Your basic appearance is a feature, you only have to pay points to change what it is.

On the other hand, someone with appropriate skills (commonly naturalist, possibly also exotic skills if the trees have been transformed) should get a roll to notice an oddity.

Originally Posted by Rasputin

Especially with player characters getting suspicious whenever you ask how they act around an otherwise benign object that no GM will bother bringing up to them unless it is killer?

Either don't bring up the object (make a roll behind the screen to notice), or make a habit of bringing up red herrings.