From what Christian provided, the trust has gross proceeds but there is no "gross income" because the stocks were sold at a loss. The general definition of gross income in the tax code would include NET gains from sales of stocks, not the gross proceeds. Trusts and estates use the same definition of gross income as for individual returns, and the general definition at IRC sec 61(a)(3) says "gains derived from dealings in property" and more specifically for trusts and estates at sec 1.641(a)-2 that says this:
As an example, a person whose only income is $15,000 in social security benefits of which none is includable in AGI, that person's gross income for tax purposes using the general definition is zero. If that same person sold stock for $6,000 with basis of $4,000, the person's gross income for tax purposes would be $2,000, not $6,000.