You an pick a baby gerbil a day after birth but try not to hold it too long because mom will worry and if you give your scent to the gerbil baby then the mom could attack and maybe even kill it(eat). It is Probably best you leave the baby for a week though so it develops and is stonger and more attached to mom. If you do pick it up then make sure its over a pillow or something soft as they can jump from an early age and this can cause death!!!! Be safe with it as it is very dependant on everything around it (mom & you)!!!
You shouldnt always handle baby gerbils because they have very sensitive skin
It all depends on the parents. If they don't stress with your hand in their home with the babies there, they maybe okay with it. I handle all my newborn gerbils from the day their born. But I'm an experienced breeder, just proceed with caution. Dip your hand into their bedding so that you smell like the gerbils, and don't wear lotion/perfume/etc. That's all I can say.
You should wait ATLEAST 7 days before touching them. They have sensitive skin.