You must NEVER pick your gerbil up by her tail. If you do, you can cause her serious injury, because the thin skin on her tail can actually slip off. If this occurs, the portion of her tail without skin will need to be amputated, or the tail will slough off.
To correctly pick up your gerbil, place your hand over her back and encircle her body with your thumb and fingers. Your gerbil may become frightened if she is not held securely. If your gerbil is used to handling, encourage her to come onto your hand, and gently encircle her with your fingers and thumb as you lift her. A final option is to coax your gerbil into a tube or cup, then place your hand(s) over the open end(s). Also, do not hold your gerbil "belly up"; this is not a comfortable or typical position for her, and she will likely struggle and increase the possibility of injury.