[PatrioticAmerican]

" The following items of gross income shall be treated as income from sources(activities) within the United States"

That is the only sentence needed to be understood; "items of gross income shall be treated as income".

[mvpel]

The point is that some sources are taxable, and other sources are not taxable. You can't calculate your tax without knowing what your taxable income is.

The sentence "items of gross income shall be treated as income" says nothing about taxable income, or sources.