Is it necessary to put in a new water pump or can you just change the gasket?

If you are simply reinstalling the old, good water pump that you took off of your engine, for whatever reason, then yes, it is perfectly fine to just change the gasket and put it on.

It's generally not a great idea to install a used water pump from a different vehicle, though. Their history is questionable and the price of new waterpumps is such that you usually aren't saving enough money to be worthwhile.

Something else to consider; If the part is old, or high mileage, check around for the price of a new one. Some waterpumps are really inexpensive, so if you can get a new one inexpensively, you'd probably save yourself a few bucks, in the long term, if you replaced it now.