Can you hold a gerbil by its tail?

I do not think that you can walk a gerbil. you would need a whole body harness small enough for a gerbil, and even then they will chew through it or slip out of it. Gerbil are masters of escape. not to mention that if you were to walk a gerbil, they would get all dirty and get fleas, and if you saw them walk out of your sight into a bush while on the walk, they may get squished by something.

Honestly, what I am trying to say in a whole round-about way, It is not a good idea to walk gerbils.

I really cannot think of any Pros to walking a gerbil, only cons. now, if you have a whole play pin set up for them in a room where no other animals are, and make sure that they cannot get out of it, then you can put them in that and let them run around your room.