Gross Income Multiplier is the ratio of the price of a real estate investment property to its annual rental income before expenses. Therefore giving you how many years of rent income it would take to payoff the property. For real estate investors the lower this number is the better of an investment the property seems to be.

For Example:

Gross rental income = $25,000
Vacancy = (10%) = $2,500
Property Price = $300,000

Therefore your Effective Gross Income on the property is $25,000 - $2,500 = 22,500

Your Gross Income Multiplier is $300,000 / $22,500 = 13.3

The Gross Income Multiplier is often called Gross Rent Multiplier (GRM)