Gabara gets its name from Ichiro's bully, who is also called Gabara.
The audio commentary for Classic Media's 2008 DVD release of All Monsters Attack humorously compares Gabara to a bipedal cat with car engine problems. Although not based on any real animal, living or extinct, Gabara is very similar to—and more than likely based on—the Oni of Japanese mythology and folklore. His costume design is very reminiscent of these demonic ogre / troll-like beings, further mixing the turquoise scales of a reptile with orange, cat-like fur. He also has three horns on his head, again much like the Oni, whose horns varied between one-to-three on average.
Gabara's ability to electrocute things on contact is possibly a reference to Oni lore, as they too were sometimes associated with the natural forces of lighting and thunder.
Just as All Monsters Attack is one of the more distinctive entries in the Godzilla canon, Gabara is very unique amongst the other Toho monsters. It seems that Gabara exists entirely inside the dreams of lead child character Ichiro.
In the context of the film, Gabara is a resident of Ichiro's imaginary Monster Island, and a constant tormentor to the much smaller Minilla, inspired by Ichiro's own real life problems with bullies around his neighborhood. After receiving combat training from his father Godzilla, and extra assistance from the human protagonist, Minilla does successfully challenge it. Finally, Gabara was confronted by Godzilla himself and got beat up. Gabara retreated after Godzilla performed Seoi nage on it.
Despite the film's overuse of stock footage, Gabara was the only original monster created for the film, along with newly filmed fight scenes and effects.
Gabara did not make any further appearances in the Showa Godzilla films, but it did make a 'guest spot' in the fourteenth episode of the short giant hero television series Ike! Greenman. However, it's unclear if this means Gabara exists outside of Ichiro's dreams, that Greenman's adventures (or at least that episode) take place within them, or if the two are unrelated.
In a 1990's Monstervision commercial for Turner Network Television (TNT), which used Godzilla film footage with Blue Oyster Cult's related song in a music video-like fashion, Gabara is incorrectly referred to as "Baragon."
In the Simpsons episode "30 minutes over Tokyo,' in the end Gamera, Rodan, Mothra and Godzilla are attacking the Simpsons and Gabara's laugh-like roar is heard.
Within the context of the film, Gabara is a representation within the dreams of Ichiro, the main character, of a schoolyard bully of the same name. Similarly, Ichiro is represented by Minilla, Godzilla's son. After Godzilla defends his son in Ichiro's dreams, Ichiro is able to overcome his fears and confront the real Gabara.
Gabara is the only monster in the Toho universe that exists only in dreams; that is to say, it does not exist in the "real" world like Toho's other kaiju.
Gabara is probably an imaginative take on the Oni, an ogre-like race in Japanese mythology. Confirmed kaiju that are Oni can often be seen in the Ultraman franchise, with characters like Oni-On, from episode 27 of Ultraman Leo, and the Sakuna Oni from episode 16 of Ultraman Tiga.