This is simple. at the end of the year, you have an amount of revenue from the property. Let's say it is $100,000. This is what your accountant would arrive at, and includes late fees, laundry income, lost rent from an eviction, etc. It is what your accoutant would put as "revenue" on your tax return. You simply take this figure and multiply it by a GRM to get the value. Let's assume a GRM of 6. In this example, 6 x 100,000 is $600,000. This is what the property is worth using the GRM method.
Typically if you're dealing with a small residential income property (2-4 units), you would use the Gross Rent Multiplier (GRM) to determine overall value of a property based on gross market rents of similar units in the area. Generally in Los Angeles city, becasue of rent control restrictions, or high percentages of owner occupied units, the GRM method is given secondary weight to the sales comparison approach in determining market value, even for 2-4 unit properties.