only differences between GIR and regular SIR units are the unique coloration of the luminous parts of GIR's body (teal instead of red), a different configuration for the eyes, and his cross-hatched mouth. Being made from garbage might mean that GIR is an out-of-date model of the SIR unit. His disguise is a green dog suit with a zipper on the stomach and a head that can be pulled over like a hood. It also includes a small tail that he can somehow make it wag like a human dog. The eyes sometimes blink while other times it doesn't. Also, the mouth sometimes opens when he talks, whereas other times it stays closed while he speaks. He also has a small child disguise with blond hair and the mouth hanging open so he can see, but it was only used in two episodes.
GIR is extremely hyperactive, and will eat just about anything, regardless of whether it's edible or not. Most of the time he spouts nonsense, but does have a few, short moments of clarity; in Walk for Your Lives, in which he contemplates if making the explosion go faster will make things worse ("But if the big 'splody goes fast, won't it get all bad?"), and in Bad, Bad Rubber Piggy, when he questions the wisdom of sending a machine into the past to destroy Dib. He also commmonly shows brief lucidity when he obeys and carries out Zim's commands, during which the luminous parts of his body will often flash in red, the color they are in normal SIR units. Unlike Zim, GIR doesn't mind getting involved with typical Earth culture; in some episodes he even admits to loving Earth. GIR also has a strong love of Earth snacks, such as tacos, taquitos, candy, chocolate bubblegum, waffles, cupcakes, tuna, biscuits, chicken with mayo, pizza, Suck-Monkies, numerous Poop products , mashed potatoes, muffins and corn. He can eat and enjoy them, even though he is a robot. He is also shown becoming fat in Halloween Spectacular of Spooky Doom after stealing all of the Halloween candy from trick-or-treaters. Despite his sub-par disguises, GIR is actually quite good at blending in with the humans due to his behavior being similar to theirs. In Attack of the Saucer Morons, he even befriends multiple humans, all of them females.
His favorite assignment from Zim is "monitoring Earth broadcasts", or basically watching TV. When doing so, he usually watches The Scary Monkey Show, which he declares to be his "favorite show". His favorite movie is Intestines of War, which he has seen over 100 times, as evidenced by the episode FBI Warning of Doom. He's also very loyal to Zim, despite disobeying him often and mainly being a hindrance to his plans. There are times where Dib "tricks" him into doing things, like putting a camera in Zim's base in Zim Eats Waffles, and even inviting him into the house in The Frycook What Came From All That Space. This is probably because Dib doesn't seem to bother GIR as much as he does Zim, saying in Planet Jackers that "he seems nice".