Gross income is all income you receive in the form of money, goods, property, and services that isn't exempt from tax.
That has always been a trick question. There is no exemption for Social Security benefits in the Internal Revenue Code. In fact, Section 86 starts out saying that they are included, before going into some exceptions. Does one parent have $50,000 income and is filing separately? Then the other parent, whose only income may be $12,000 Social Security, has too much AGI to be claimed as a dependent.