My mother died last year and she left me her house which was under a reverse
Mortgage.
I plan to sell the house, pay off the reverse mortgage and keep the remaining
Profit.
What percentage capital gains would be applied to the profit from the house
Sale?

1. Your cost basis of the house is the fair market value of the house on the date your mother died. So you may or may not have a profit. If you have a loss, the loss is not deductible.
2. If your mother owned the house and lived in it for 2 years in last five years, you are eligible to exclude gain of up to $250,00. In this case the cost basis is your mother's cost basis plus cost of improvements. Your U.S. Tax Return: Profit From the Sale of Your Home