For SA residents: "Gross income" means the total amount of worldwide income that you earned during the tax year, excluding income that is of a capital nature.
Note that "income" is also money that is owed to you for work you performed, even if not paid to you yet - for example payment for a service that you performed.
Non SA resident: "Gross income" is the same as above, but limited to income from SA sources only.
Income of a capital nature is usually money derived from selling your possession, e.g. sale of your house.
Please also see the following related definitions: Income and taxable income.
Do Your Tax Return Easily