It's all the money you have coming in during a 12-month period.
If you get a paycheck, it's the largest amount--the amount before anything's deducted for federal income tax, state income tax, local taxes, social security, etc.
If you get paid by the hour, it's the total of what you were paid.
If you carve duck decoys, it's the amount that people paid you when you sold them. It wouldn't count your expenses--wood, advertising, paint, etc.
Gross income actually is pretty simple to calculate.
Hope that helps.