I can imagine numerous ways to measure the efficiency of a cooker.
In general, to measure the efficiency of anything, you divide what comes out of the device by what goes into it. In the case of a cooker, you would divide the amount of energy that comes out of it by the amount you put into the cooker.
In the article I have cited here, the input to the solar cooker is calculated by the area of the collector and the amount of solar energy received per unit area. The output is determined by measuring the time required to boil off a known quantity of water. I don't know if this is the method you plan to use to determine the efficiency of your solar cooker or not, but I'm sure you're using something similar.
You can use a variation of this for a conventional gas grill. The output energy would be measured just the same way you measure it for the solar grill - boiling away a known amount of water. For the input energy, it would be reasonable to assume (if the grill is fairly new) that the input energy is that specified by the manufacturer. In this web site from Weber, you can see the the INPUT is specified as 38,000 Btu (British Thermal Units) per hour. All grills have similar ratings. As grills get older, the burners get corroded, or clogged with cooking remnants, and don't flow as much gas. So, in that case, the input Btu would drop some. In a laboratory, engineers putb flow meters on the gas supply lines and measure the amount of gas going into the grill. They convert that to Btu per hour to get an accurate measurement of the true input energy. I don't recommend attempting to measure the actual gas flow as a 10th grade science project. It requires special equipment - EXCEPT if you have a very accurate scale, and are using a propane fuel grill, you could measure the weight of propane used over the time you boiled away the water and convert that to input Btu's.
Weber Grill:http://www.weber.com/explore/grills/gen ... esis-e-330
I hope these ideas help and that you HAVE FUN with your project.