Grommet = Rubber Doughnut.
If I remember correctly, it was no more than 15 dollars at the local dealership. It is almost mandatory that you get a new one, because the old ones almost always leak once you drop the tank.
Just ask the parts dept. for a "Fuel Tank Filler Neck Grommet" and the year/model of your car. They will know exactly what you are talking about (assuming reasonable personnel knowledge).