Only the taxable amount of Social Security is considered "gross income" - generally in IRS publications, "gross income" should be read as "gross taxable income".
You will need to do a bit of calculation to determine how much of your Social Security is taxable. See below:
"Gross income. This includes all income you receive in the form of money, goods, property, and services that isn't exempt from tax. It also includes income from sources outside the United States or from the sale of your main home (even if you can exclude all or part of it). Include part of your social security benefits if:
1. You were married, filing a separate return, and you lived with your spouse at any time during 2016; or
2. Half of your social security benefits plus your other gross income and any tax-exempt interest is more than $25,000 ($32,000 if married filing jointly).
If either (1) or (2) applies, see the instructions for Form 1040 or 1040A, or Pub. 915, Social Security and Equivalent Railroad Retirement Benefits, to figure the social security benefits you must include in gross income. See Publication 17 under "Do I have to file a return?"
