A GRM is a Gross Rent Multiplier, or a ratio of sale price to rent.
Gross Rent Multiplier: Sale Price / Rent = GRM.
Rent * GRM = Assessed Value
An example would be: 521 Market St. sold for $35,000 in 2011 and currently rents for a market rent of $500. The annual GRM for this property would be $35,000 / 500 = 70 or a monthly GRM of: ($35,000/500)/12=5.83.