Bloody Gir is a single image of Gir covered in Blood.There's a short story behind this egg. Jhonen Vasquez, the twenty-something creator of Invader Zim, was apparently going to use an image in the show of the lovable GIR with blood apparently sprayed on him. Well, Nick didn't like this very much, so they told him to take it out and refused to use it, although the image itself had already been drawn. So instead, out of spite, Jhonen hid the image in single frames throughout many of the episodes, starting with Bad, Bad Rubber Piggy.
Just about every episode afterwards contains the image in a single frame. You need a frame-by-frame VCR to spot him.
As I know, he appears in about 14 episodes. Below is a list of episodes he is said to be in. You can join the hunt too. Just grab your Invader Zim DVD sets and go frame by frame and find one for us. Good Luck :D
On a side note, Bloody Gir is NOT the reason Zim was cancelled. In fact, Nick found out about Bloody Gir AFTER it's cancellation.