The density rating on gel memory foam tells you how much a 12-inch cube weighs. For a mattress or topper, a density of 3 to 4 pounds is usually best.Some users find that products with a density rating lower than 3 may not be very durable.Gel memory foam toppers or mattresses with a density rating of 5 or higher may get too warm for people who get hot when they sleep.he density rating on gel memory foam tells you how much a 12-inch cube weighs. For a mattress or topper, a density of 3 to 4 pounds is usually best.