Cap gains and other investment income does not qualify for IRA contributions. You must have taxable compensation (earned income) to make any kind of regular IRA contribution. But you do not need to have taxable compensation to convert some amount from your TIRA to a Roth IRA.
Since cap gains do not qualify, it makes no difference if they are ST, LT, or what the tax rate on those gains is.
Alan is correct. First, to be eligible to make a Roth IRA contribution you must have earned income (compensation). Second, even if you have compensation, you can’t make a contribution if your modified AGI exceeds the limit for your filing status. Modified AGI includes all your income, including capital gains, except income from a Roth IRA conversion. See page 39 of Publication 590-A at https://www.irs.gov/pub/irs-prior/p590a–2019.pdf.