Gabe is an oversized cyberdog found in the X-8 research center's testing facility in 2281. He is bigger than the other usual police or military cyberdog, and is more powerful due to a huge intake of Psycho.
Gabe was the pet of Doctor Borous before the great war. Gabe's savage nature comes as a result of Borous feeding him excessive amounts of the chem Psycho in his dog food. This could explain Gabe's massive size (standing much taller than the Courier on all fours) as well as his hostility towards the Courier.
After the completion of A Brain's Best Friend, Doctor Borous reveals that Gabe was his only friend during his rough childhood, and was the only loyal animal when he was working at the Big MT, saying that while he was talking about Richie Marcus and Betsy Bright, Gabe would sit by his side with his head on his leg, and after mutating and experimenting on him so much shows remorse for his actions.
A Brain's Best Friend: You do not have to put Gabe down yourself before talking to Borous about it. You can avoid getting Gabe's blood on your hands by running or sneaking past him and digging up the holodisk with Gabe's bark on it. Doctor Mobius' robotic scorpions will appear and proceed to kill Gabe, but you can save him if you move quickly. Borous will thank you for saving Gabe, but there does not appear to be any reward. Note: The recording can be in any one of the digging spots and will most likely move if a save before entering the Residential Test Area is loaded.
At the back of House #103 in Higgs Village you will find Gabe's dog house. If you have the Wild Wasteland trait, approaching the dog house will cause a miniature deathclaw named Stripe to spawn; Stripe is hostile towards the player and does as much damage as the legendary deathclaw.
Upon death, Gabe will become atomic and detonate, this is accompanied by a less-than-accurate countdown announced by the Think Tank and Mobius. By running as far away as possible from the body you can easily avoid any HP loss.
Gabe is one of the least perceptive enemies in Fallout: New Vegas, with a whopping 0 Perception, the same as all ghost people, allowing the Courier to easily sneak past him to find Gabe's Bark.