U.S. citizens and residents must pay tax on their worldwide income. There is a limited exclusion for earned income (up to $103,900 in 2018), but there is no exclusion for gain on the sale of property abroad.
Depreciable property used in a trade or business and held for more than a year. All Section 1231 gains and losses are netted; a net gain is treated as capital gain, a net loss as an ordinary loss.