What is "small"? 2,3,4,5,6, or 7 figures?
Capital gains? Short or long term? Already devalued assets and have a big recovery?
Reinvest in like business to avoid the tax side?
Will this be treated as income and now your tax advantaged account contributions are in trouble?
If you have 27 years on your mortgage and are considering paying it off, how can you compare that to a down payment on a rental UNLESS that rental property is significantly greater than you primary residence.
Sorry, but the vague details probably explain the 50+ views and no responses.