While it is not normal, it does happen. The reason could be from a number of things. Not enough calcium, injury or teeth too long to name a few. The following link gives more reasons, and goes into detail:
http://www.erodent.co.uk/GerbilHealth.ht ... It is possible your gerbil's teeth will grow back if they fall out. Make sure (s) he has a diet with enough of the vitamins listed, and get the teeth trimmed by an exotic vet if they get too long. I will give you one more link. It gives other ways to keep the teeth trim. It is as follows:
http://www.ehow.com/how_2141375_keep-ger ... Good luck with your gerbil. (S) He is likely to be just fine,
... no and how did you discover the gerbil had loose teeth? If you pulled on them yeah you ripped the connective tissue.
If you mean is it normal for a gerbil to have lost a top tooth, well it depends on age and whats going on with them.
