I am a Maryland resident and recently sold an investment property that is loacted in North Carolina. The sale created a long-term capital gain. Do I have to pay a tax on the gain in both states?

Initially, yes.  You pay tax to the nonresident state on income sources within that state.  You pay taxes to your resident state on all income.  However, your resident state MD offers a credit for taxes paid to another state to mitigate the double taxation.  

After completing your federal interview you need to work your NC nonresident interview and allocate to NC only the income that belongs to NC.  At the end take note of the income taxed and the income tax; you will need that on your MD resident return.  

Now you can complete your MD resident return. In the "Credit" section you will find the opportunity to claim the credit for taxes paid to another state.