There are several types of litter comercially available. First of all, you should know that both cedar and pine litter are very dangerous to gerbils and small rodents in general. These bedding contain some compounds called phenols that cause respiratory problems in rodents. Phenols are the compounds that give these two beddings it's nice smell. You should never use pine or cedar as litter for your small rodent. Some safe litters available in pet stores are aspen, corncob, carefresh and yesterday news. You can also use plain white computer paper, paper towels, tissues or toilet paper as bedding, but these do little to nothing about smell control.
Gerbils love to dig, so when placing litter in their cages, be generous and give them 3-4 inches of it. Avoid newspaper, as your gerbils could be allergic to the ink.