Fighting
There are two kinds of fighting, boxing and balling. Boxing covers a broad spectrum, of pushing, shoving, and batting. They may use one or two paws to deflect with, they may throw their whole body into it too. Overall there are many healthy ways a gerbil can assert their position, and to stop another gerbil from doing something they don’t want to do. I’ve also seen gerbils box over who was going to be groomed. There may be squeaking and running involved, but the running should not give way to being chased.
However, when they are truly fighting, they roll up into a ball and attack each other with tooth and nail, and draw blood. Once blood has been drawn these two gerbils most be separated. They will eventually kill each other. In the wild, fights don't usually reach the fatality point as the loser will usually run away before the fight gets too serious, in a cage there isn’t anywhere to run. So when they do run, it turns into a chase. If your gerbil is fast enough they can avoid getting into another ball fight. Often during these chases the aggressor will bite the gerbil as they runaway, leaving bits marks on their butt and tail as they try to drive the other gerbil off.
Once gerbils have fought, do not attempt to put them back together. Just because you think they are cute together, or that they should be best friends, doesn’t mean that they think that. What ever issues they were having escalated into a full blown fight, where serious attempts were made on each others lives. They are not friends. Separate them. For many people this may mean needing to buy a second cage, which can be pricey. After the gerbils have healed you may want to consider finding pups to be their new friends.