They say the gross amount goes in line 6a and then (deducting the repayment) the net amount in 6b. I've been through the steps in Turbo Tax for entering SSA 1099 and there only seems to be a question for the net amount.
The lines 6a and 6b are on the actual 1040 tax return, not when you enter the SSA-1099. You only enter box 5 and any Medicare deductions and federal withholding in box 6 from the SSA-1099.
Enter a SSA-1099, SSA-1099-SM or RRB-1099 under
Federal Taxes (Personal for Home&Business)
Wages and Income
Then scroll down to Retirement Plans and Social Security
Then the second line - Social Security (SSA-1099. RRB-1099) - click the Start or Revisit button
Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0