The use of a Waterpik to clean your teeth should not damage your teeth or gums.
That is unless you have really sensitive teeth or gums and if you do have more sensitive teeth or sensitive gums then it could be possible for the Waterpik to damage your teeth and gums.
I use a Waterpik myself and have not had any problems or damage done to my teeth or gums by the use of the Waterpik.
I also make sure to brush my teeth daily and even use a regular string floss to floss my teeth and use the Waterpik every few days or so.
The Waterpik is safe to use on your teeth and for your gums and should cause you no harm.
If you notice bleeding of the gums or problems with your teeth when using the Waterpik then you could have some other problems such as gum disease, gingivitis etc.
Or you may be using the Waterpik wrong.