In every cage there is a dominant and a submissive gerbil. Here are two behaviors which will show that will show where a gerbil is being dominant.
Grooming: where one gerbil will groom another. This is a strange one to see as you may not expect dominance to be shown in this way. The dominant gerbil will nibble and wash the fur of a submissive. On introduction the dominant gerbil will always groom the submissive gerbil first immediately after being introduced. It is done to make the submissive feel part of the family.
Mounting: A way one gerbil might show dominance over another is by mounting them. They will try and piggyback the submissive and basically follow them around.