Arrange egg roll wrappers, water, sauerkraut, corned beef, and Gruyere cheese in an assembly line in a work space large enough to easily accommodate everything without being crowded.
Take one wrapper and place in front of you with one corner pointing at you. Dip fingers into water and use them to wet the side of the wrapper facing upwards.
Place small, even amounts of sauerkraut, corned beef, and Gruyere cheese, respectively, onto the third of the wrapper nearest you. Fold the corner opposite you over the fillings and roll slightly to surround the filling with wrapper. Fold the sides to enclose the fillings completely, rolling to seal completely. Put completed rolls on a tray. Repeat until all ingredients are used. Refrigerate rolls until oil is heated.
Heat oil in a deep-fryer or large saucepan to 350 degrees F (175 degrees C).
Fry chilled egg rolls in hot oil in batches until golden brown, about 5 minutes per batch.