He was seen in the episode, The Secret, when Dwight went to Oscar's house to see if Oscar was lying about being sick. Even though they are obviously in a relationship, Dwight is oblivious and spends time watching a movie with them.
Gil is once again seen in the episode, A Benihana Christmas, but Oscar and Gil leave right after getting a look at the Christmas party
Oscar mentions in Beach Games that he is planning to break up with Gil.
In Niagara Part 1, Kevin is mistaken for Gil while being introduced to Pam's relative.