If he bought the property, then we need to know the purchase price plus the cost of any improvements made to determine your father's basis in the property. The gain would be the sales price (less selling expenses) minus his basis.
Assuming the property is improved (i.e., has some sort of structure), part of the gain (if any) would be Section 1250 gain (depreciation recapture over the 20-year period of rental) which would be taxed at ordinary income tax rates (up to a maximum of 25%) while the rest of the gain would be taxed at the long-term capital gains tax rate (up to a maximum o 20%). There is also the NIIT of 3.8% depending upon whether his net investment income exceeds $200,000.
