Gerbils like a place to have a private place where they can sleep and hide.
If you provide lots of bedding this isn't nesassary, as they can make there own burrows to nest in, however, if your gerbils don't have enough bedding to burrow in, you should give them a hut.
There are many different types of nests, generally wood will have to be replaced more often, as it will get chewed, and may start getting damp from urine. Cardboard boxes and edible logs will need to be replaced very often, as they will get chewed to bits in no time.If you want a more permanent hut, you may consider a ceramic one.
Give gerbils unscented toilet tissues to line there nests. Paper towels work to, but they aren't nearly as soft, and pups legs can get wrapped around them and lose circulation.
Never use nesting fluff sold at the pet store, or cotton balls. these can be a serious choking hazard, and can get wrapped around feet. they can also get wrapped around intestines, and lead to death.