If you received a 1099-B with stock/fund sales, then you'll need to include that on your return. If you've already filed, then you'll have to amend your return and add the missing info.
It's not the gain/loss amount which is the issue, it's the gross proceeds amount. If you don't include those sales, IRS will look at the proceeds (the total of all your sale prices) and assess you tax as if that were your gain. Even though your actual gain (once purchase price has been subtracted) may only be a hundred bucks, you'll wind up getting an IRS notice showing a much higher gain, plus a tax bill and penalty & interest.
You need to amend and list that 1099-B info.
