NO do not use those ear drops........

If he does have something wrong with his ears he needs anti-biotics, I would suggest oral baytril (given directly into the mouth not into the water).

You need to get to the vet asap, delaying will mean he gets worse meaning that treatment is going to cost you a lot more and chances of success are less.

Your mum shouldn't have to say whether or not he can go to the vet, you were able to get the bus to the store, pay for the gerbil and get him home. They have a vet at that shop so even without your mums help you can get him to the vet.

As has been said about rats is true for gerbils, there comes a point when the damage to the ear is not treatable.

Please get him to the vet tomorrow.