From page 48, https://www.tax.ny.gov/pdf/current_forms/it/it203i.pdf :
If you moved out of New York State, you must accrue any item of income, gain, loss, or deduction that, under an accrual method of accounting, would be reportable at the time you changed your residence.
Since an unrealized capital gain would not be reportable at the time a taxpayer moved out of NY, I do not believe it would be allocable to NY on a part-year resident return.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
