The seal is a closed cell foam (non absorbent) with a self-adhesive backing. See also the cover plate, AlfaStop item # 1MW160.
 
Part Name: Fuel (Gas) Tank Sender (Sensor) Gauge Cover Plate Seal
AlfaStop Product Code: 1RU205
Original Alfa Romeo Part No: 
Model: Guilia 101 Series, Giulietta 750 & 101
Quantity Per Car: 1