Currently, the realized gain or loss line is either blank, the brokerage account, or the correct gain/loss category.

Where do I access input field to have the realized gain/loss default to a designated account?

Then go over to the right side of the screen (account properties) and move over to the tax code column. Choose the cell for your new gain account, use the drop down menu to choose your tax code and click on the appropriate tax code.

OR, you can also change your tax code by:

Accounts | Tax Code Changes and choose which tax effect you would like your new account to have.

Then go to Enter Transations. Once you are ready to enter a transaction, go to Tools | Transaction Activity Options.

For each gain option (short,mid,long) click on the button and when the screen with the choices pops up, chose the new gain account you set up. It should be listed with the other default choices.

Once you have done that, any sale you post should auto default to that new gain account you set up.