I got a schedule K-1 (form 1065). I have some long-term gains in 9a, which appear as distributions, too, in 19A. What should be the income tax rate on this?

9a is taxable as long-term gain and increases your tax basis. 19A is not taxable income but reduces your tax basis. why 20%? more income can have an affect on other items.  you would need to review the cap gain and qualified dividend worksheet to see how the gain is taxed. 

To provide a little more clarification, the capital gains rate can be 0%, 15% or 20%.

Your rate depends on your taxable income and filing status.

As was noted by @Mike9241 , you can see this determination by reviewing your capital gain tax worksheet within TT.