The Title 26 definition of 'Gross income' is not a legal definition of 'income'. Nor have you shown where in Title 26 it legally defines 'gain' in regards to the individual.
The only way 'gain' can be applied to the individual is if they regarded as a corporation..as that is the only defined use of taxable gains in Title 26.
So which is it?
Are we being taxed as private individuals under some hidden legal definition?
Or are we being taxed under Title 26 as corporations?