In order to input gains from investments, you would first need to determine the type of investment that you are looking to claim. You may be issued a T-slip associated with that investment type and you will report that income slip on the corresponding page in TurboTax. Capital gains could be reported on a T3, T5 or T5008 slip.

For example, if you want to report interest income from a savings account, you would need to file a T5 form provided by your bank.

For more information, you can refer to these links:

My CRA- Investment Income 

Taxing Investments in Canada