Form 1099-B

The TurboTax system should calculate the gain or loss that you have using the information that you are entering even if your brokerage chose not to calculate that for you.  So that part is fine.

If you have trades on your 1099-B that don't have any basis then that may mean that the broker didn't know when you bought those stocks or how much you paid for them.  That can happen when you're selling off things that you have had for a long time.  

If you aren't sure when you bought them or how much you paid either then it is best to contact the broker who issued you the 1099-B and see if they have any clues that they can offer to help you find the cost basis.

@XXSHNAYXX 

It is possible you are dealing with a "wash sale", where you have a cost greater than the sales proceeds, in that case the net gain or loss would be $0. If so, you need to indicate that when you enter the sale in TurboTax, or else it will show a loss when it should be $0. To do so, when you enter your investment sale, click on the box that says The cost basis is incorrect or missing on my Form 1099-B and you will see box 1(g) where you can enter the wash sale amount.  

Otherwise, the proper gain or loss would be the sales proceeds less the cost of the investment. If you entered that correctly in TurboTax and you see those numbers reflected properly on your screen, then the gain or loss be would correct.

For type of gain, look at the date acquired and date sold to see if the sale is long term or short term. Long term is more than one year,  short term is one year or less. As long as you get that right you should be OK.