@richardR1 fully agreeing with my colleague @Opus 17 ( for a very complete answer) , just wanted to add that the double taxation relief ( through foreign tax credit) is based on "double Taxation " clause in the tax treaty between US and that other country. Thus if the country where you are paying capital gains tax does NOT have a tax treaty with the US, then there is no real mechanism for claiming foreign tax credit -- form 1116 does ask for foreign income source country. However, if you itemize and use the foreign taxes paid in place of State & Local Taxes -- SALT ( with the current US$10,000 limit ) there is no such question. It is somewhat of an uncharted territory -- I have not seen any case law (tax court) on this, yet.
Also please note that because foreign currency etc. is involved, any financial accounts that you own and / or have signature authority over may come under FBAR ( FinCen form 114 on line only ) and FATCA ( form 8938 ( filed with your return ) may come into play. While these are information only, have no tax implications, willful violation can attract onerous penalties.
pk
