ADJINC (adjusted gross income) includes income from capital gains and is available in the CPS ASEC from 1992 to the present. Tax variables are based on the Census Bureau’s tax model which estimates federal, state, and payroll taxes based on simulated individual tax returns (for more information the CPS Tax Model, see this paper summarizing the post-2004 model - there is an overview of adjusted gross income on page 9 that you might find useful).
In addition, CAPGAIN & CAPLOSS are two variables available in the ASEC from 1992 to 2008 that contain information on pre-tax capital gains and loss accrued by an individual. This forum post contains a discussion of these two variables and why they were discontinued by the CPS.