As already said, gerbils are highly territorial animals. The best way of introducing two unknown gerbils is by the split cage method. To do this, you'll need to create a division in the cage. This division has to be made of wire mesh, so that the gerbils can see, smell and somewhat touch each other, but that they can't harm each other. You have to be careful and use wire mesh small enough so that they can't put their fingers or tails through, cause they might end up losing them. Here is a picture of what a split cage should look like:

After you have the split cage set up, you place one gerbil on each side of it. At least once a day, you should switch them from side. After a couple of weeks they should have gotten used to each other. You'll know when they are ready for a direct introduction when you see them curious/friendly about each other. Signs of these are that they try to groom each other through the division and that they sleep next to each other. When you think it's time to do a direct introduction, you should do it in a neutral area. This is an area in which neither of the gerbils have been before, they don't need to defend a territory that is not its own. I advise you to use gloves in case that a fight arises, as gerbil bites can be painful. Some tricks you can use to help in the introduction include putting some food in the place of introduction (if they are eating, they can't fight), and putting a dab of vanilla extract on each gerbil's noses and bums (gerbils recognize each other from smell, vanilla extract disguises their smell). If all goes well in the neutral territory, clean their cage throughly, including toys, and place them in the cage. Watch them for an hour or so, to make sure that they don't fight all of the sudden. Once they sleep together, you'll know for sure that your gerbils are friends with each other.