GIR's Big Day is one of the many unfinished episodes.
It begins with Zim ordering GIR to help clean the yard, and GIR complies. As GIR is cleaning, a truck driving by somehow manages to hit him. The truck driver, having expected the "dog" to be dead, was surprised to find that it was just fine. In excitement he told others of the accident and soon enough a crowd had gathered. After a short time, some scientists took GIR to their lab to determine how the "dog" survived. While they were in the process of cloning him, some doughnut crumbs got mixed into the DNA, which caused GIR to morph into a giant doughnut-dog monster.
Clearly, chaos ensued, though the exact details of the plot beyond that are unknown.