You don't pay capital gains. You receive capital gains. Capital gain is the profit that you make when you sell an asset. The capital gain is income. You pay income tax on the capital gain.
You pay tax on the gain from selling your primary residence in the year that you sell it. What you do with the money, what other homes you own, or where you move to, have no effect on when you pay the tax or how much tax you pay. As Bsch4477 pointed out, if you meet all the requirements you might qualify to exclude from your taxable income part or all of the gain from selling your primary residence. The requirements for the exclusion do not include what other homes you own, where you move to, or what you do with the money from the sale. If you have to report the sale (even if you qualify for the exclusion of gain), you report it on your tax return for the year that you sold the residence.
If you have a large taxable gain on the sale, you should make an estimated tax payment instead of waiting to pay the tax until you file your tax return.
