It is a gelatin cake made to resemble Gary the Snail. It contains pink gelatin for his shell, blue for his eye antennas and edges of his body, and green for his eyeballs and belly. The only difference in appearance between this cake and the actual Gary is that it doesn't contain the spots and spirals on his shell or pupils in his eyes. The plate it's served on contains decorated pink and orange flowers around the gelatin.
Role in episode
SpongeBob is visiting his grandmother to celebrate her birthday and leaves Patrick in charge of Gary while he is at his grandma's house. Although SpongeBob feels as though he should be the one serving his grandma since it's her birthday, the latter offers him a dessert: a gelatin cake of her grandson's pet snail, Gary. SpongeBob appreciates it, though the thought of Gary being in untrustworthy hands leaves SpongeBob nervous, so he uses his grandma's house phone to call Patrick and see if everything's alright.
Because SpongeBob rushes to his grandma's house phone to call Patrick just after being served the cake, he is not shown eating it on-screen.