Yes, you are correct. Only enter the amount of capital gains from North Dakota source income. Capital gains are included as taxable income in North Dakota; however long-term gains may be eligible for the North Dakota net long-term gain exclusion. The exclusion is equal to 40% of your net long-term gains.
From North Dakota's instructions:
If your federal taxable income includes a net long-term capital gain (including a capital gain distribution from a mutual fund), you may be able to exclude 40 percent of the gain from your North Dakota taxable income. If you were a full-year nonresident or a part-year resident of North Dakota for the year, only a net long-term capital gain reportable to North Dakota is eligible for the exclusion.
North Dakota Instructions on page 12:
https://www.nd.gov/tax/data/upfiles/media/2018-individual-income-tax-booklet_1.pdf?20190405164300
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