Your question raises some interesting questions. Depending on the value of the property, this might be a good time to consult a qualified US tax advisor such as a CPA with experience in reporting these kinds of sales (not necessarily every CPA or other tax advisor.)
1) Is the sale of property located outside the US subject to capital gains tax? Yes, it is. US citizens are subject to income tax based on their worldwide income. There are various rules and exemptions, but the Foreign Earned Income Credit does not likely apply since it is only for earned income, not capital gains. If the sale took place in the US there would be reporting requirements at the time of the sale. For sales outside the US, one would still be required to report the sale and pay any tax due on the income tax return for that year. Note that this might also require paying tax close to the time of the sale to avoid any underpayment penalties, although there are also rules and exceptions for that.
2) What is the basis of property gifted to you? You don't pay tax on the basis, only on your gain (or loss) from that basis. The IRS has rules for calculating this, including the FMV (fair market value) and the taxable basis of the donor (person who gave this to you) at the time of the gift. "To figure the basis of property you receive as a gift, you must know its adjusted basis (defined earlier) to the donor just before it was given to you, its FMV at the time it was given to you, and any gift tax paid on it. [Publ 551, Basis other than cost, Property received as a gift]"
You can read more at...
https://www.irs.gov/publications/p551/ar02.html#en_US_201412_publink1000256961
3) What is the tax rate on capital gains? The tax rate varies based on your income, but in some cases it is lower than your general tax rate, and in some cases it is zero. So the tax rate depends on both your total income and your capital gain tax rate, not just your overall tax rate, as well as the amount of your gain after you exclude your basis in the property. While the rate can be as low as 0%, in many cases the federal capital gains tax rate is 15%. There may also be state capital gains taxes depending on your US state of residence. Also foreign taxes you pay on the transaction are likely deductible, so you want to track them and look into that as well.
4) Does anything happen to your basis when you become a US citizen after owning the property? I actually didn't know the answer to this interesting question. I did find a couple blog posts discussing nuances of these situations. Apparently your basis does not change except in limited instances. For instance, http://hodgen.com/property-you-acquired-before-coming-to-the-usa/, and http://www.bankrate.com/finance/taxes/calculate-cost-basis-indian-home.aspx (includes a good discussion of a related example), and https://www.greenbacktaxservices.com/blog/expat-taxes-buying-selling-real-estate-abroad/ (does not discuss the citizenship issue, but notes other issues regarding selling foreign properties.)
Doesn't really matter when it was gifted to you. The main thing is, you have no money of yours that was put into it, or it's acquisition. Therefore you can expect to be taxed on 100% of the gain. Yes, you will have a capital gain. The taxrate will depend on your entire worldwide income for the tax year, and can be as high as 39% taxed on your entire worldwide income for the tax year. See the IRS tax rate schedule at <a rel="nofollow" target="_blank" href="https://individual.troweprice.com/public/Retail/Planning-&-Research/Tax-Planning/Prepare-Your-Taxes/Tax-Rate-Schedules">https://individual.troweprice.com/public/Retail/Planning-&-Research/Tax-Planning/Prepare-Your-Taxes/Tax-Rate-Schedules</a>
Your question raises some interesting questions. Depending on the value of the property, this might be a good time to consult a qualified US tax advisor such as a CPA with experience in reporting these kinds of sales (not necessarily every CPA or other tax advisor.)
1) Is the sale of property located outside the US subject to capital gains tax? Yes, it is. US citizens are subject to income tax based on their worldwide income. There are various rules and exemptions, but the Foreign Earned Income Credit does not likely apply since it is only for earned income, not capital gains. If the sale took place in the US there would be reporting requirements at the time of the sale. For sales outside the US, one would still be required to report the sale and pay any tax due on the income tax return for that year. Note that this might also require paying tax close to the time of the sale to avoid any underpayment penalties, although there are also rules and exceptions for that.
2) What is the basis of property gifted to you? You don't pay tax on the basis, only on your gain (or loss) from that basis. The IRS has rules for calculating this, including the FMV (fair market value) and the taxable basis of the donor (person who gave this to you) at the time of the gift. "To figure the basis of property you receive as a gift, you must know its adjusted basis (defined earlier) to the donor just before it was given to you, its FMV at the time it was given to you, and any gift tax paid on it. [Publ 551, Basis other than cost, Property received as a gift]"
You can read more at...
https://www.irs.gov/publications/p551/ar02.html#en_US_201412_publink1000256961
3) What is the tax rate on capital gains? The tax rate varies based on your income, but in some cases it is lower than your general tax rate, and in some cases it is zero. So the tax rate depends on both your total income and your capital gain tax rate, not just your overall tax rate, as well as the amount of your gain after you exclude your basis in the property. While the rate can be as low as 0%, in many cases the federal capital gains tax rate is 15%. There may also be state capital gains taxes depending on your US state of residence. Also foreign taxes you pay on the transaction are likely deductible, so you want to track them and look into that as well.
4) Does anything happen to your basis when you become a US citizen after owning the property? I actually didn't know the answer to this interesting question. I did find a couple blog posts discussing nuances of these situations. Apparently your basis does not change except in limited instances. For instance, http://hodgen.com/property-you-acquired-before-coming-to-the-usa/, and http://www.bankrate.com/finance/taxes/calculate-cost-basis-indian-home.aspx (includes a good discussion of a related example), and https://www.greenbacktaxservices.com/blog/expat-taxes-buying-selling-real-estate-abroad/ (does not discuss the citizenship issue, but notes other issues regarding selling foreign properties.)