A Waterpik water flosser should not damage your gums because the Waterpik does not have enough water pressure to cause damage to your gums.
However if you do have sensitive gums already then use of the Waterpik could cause some slight bleeding of the gums which is normal.
But the Waterpik itself should not cause much if any bleeding to normal healthy gums.
The Waterpik is a great dental tool for keeping your gums clean and healthy as well as your teeth clean and healthy and to help prevent gum disease, gingivitis, tooth decay etc.
I use my Waterpik water flosser every 2 to 3 days to keep my gums and teeth clean but I floss about 3 times per day with the regular string floss.
When used properly the Waterpik should not damage your gums.