There is no deferral of a capital gain. Deferring more of your compensation to a traditional 401(k) simply reduces taxable income. That doesn't change how much of a capital gain is includible in income, but a reduction in taxable income it could reduce the tax rate that applies to your capital gain if the capital gain is long-term.
It's a bit late in the year to alter your deferral to a 401(k) unless you are self-employed and elect a one-time 401(k) deferral. Unless 2023 would be the first year for a solo 401(k), the deadline to make the deferral election is year-end 2023.
