This has been discussed before, but they are closed here. When you download a transaction type, you can't change the Category, and the Budget possible items don't include LTCG or STCG. This is such an oversight because, like many, some of us use these year-end gains in our annual planning on the Revenue side.
There is a workaround that might work, but why? Please, Quicken, add gains to the budgeting tool.
The workaround is to change the downloaded gain to a Payment to an account you set up and label. Don't know what this might do to the gains calculator, which I don't use. [Edited - Readability]