Yes, the net gains are subject to NIIT.
Presumably, you are talking about line 5a on Form 8960 2022 Form 8960 (irs.gov) which says “Net gain or loss from disposition of property (see instructions)”?
If so, then the instructions 2022 Instructions for Form 8960 (irs.gov) for line 5a tell you to include • Form 1040 or 1040-SR, line 7, and Schedule 1 (Form 1040), line 4 which is where your net capital gains are reported on your 1040
Sorry, that’s not correct. From Topic No. 559 Net Investment Income Tax | Internal Revenue Service (irs.gov) (my bolding)
Definition of Net Investment Income and Modified Adjusted Gross Income
In general, net investment income for purpose of this tax, includes, but isn’t limited to:
- interest, dividends, certain annuities, royalties, and rents (unless derived in a trade or business in which the NIIT doesn’t apply),
- income derived in a trade or business which is a passive activity or trading in financial instruments or commodities, and
- net gains from the disposition of property such as stocks, bonds, mutual funds, and real estate (to the extent taken into account in computing taxable income), other than property held in a trade or business to which NIIT doesn’t apply.
- net gains from the sale of a passive partnership or S corporation ownership interests.
Edited to add: I will point out that if some/all of your capital gains are from property that was held in a trade or business to which NIIT doesn’t apply, then you should be using the Lines 5a–5d—Net Gains and Losses Worksheet in the instructions to properly account for disregarding those gains.