Gross Income

Gross income is total income before any expenses or deductions. For a rental, gross rental income is the scheduled rent at full occupancy plus recurring other income, before vacancy, taxes, insurance or management. It is the figure a gross rent multiplier divides price by, and the word gross is the whole point of the metric.

A $300,000 property renting for $2,500 a month has gross rental income of $30,000 a year and a GRM of 10. Add $50 a month of parking and the gross income is $30,600 and the GRM 9.8. Whether other income is included should be consistent across every property you compare.

Gross income is not what you keep. Vacancy, expenses and the mortgage all come out of it, and on a typical rental they take 60 to 80% of it. That is why a GRM is a screen and not a return, and why the Wikipedia article, written for US tax law, defines the term the same way a rental investor does: everything that comes in, before anything goes out.

Further reading: Gross Income on Wikipedia.