Gappa looked like a cross between a bat and a spider. They had thick, wiry fur, thin exoskeletons and the wrinkled noses of bats. They had eight bony legs. The front two had long fingers on the end. The Gappa had limited sight and used touch and echolocation to find their way around. They communicated with a telepathic field which could also be used to possess beings. Being adapted to a cold climate, the Gappa were weak in heat and could be killed in seconds by high temperatures.
Gappa could reproduce asexually by laying eggs, which were kept in the corpses of their prey. These eggs would cause the body to mutate, turning the skull into an egg-like shape. When the skulls broke down, anyone who breathed in the dust could be controlled by the Gappa. (PROSE: Snowglobe 7)
The Gappa originated on Hydropellica, but by the Stone Age of Earth, they had over-hunted their food sources and were in danger of becoming extinct. A group of Modrakanians took the last Gappa from the planet, intending to take it home and preserve it, but the ship crashed on Earth. Though the Modrakanians were killed in the crash, the Gappa was able to escape. The early humans of Earth were able to drive the Gappa into the ice using fire.
In the 2090s, the governments began moving sheets of ice into Snowglobes to deal with global warming. The section of ice containing the Gappa was moved to Snowglobe 7. The Gappa awoke and used two humans and a Flisk as hosts to breed a new group of Gappa. The skulls of the hosts were found by workers from Snowglobe 6, who believed they were eggs. The eggs were taken to the hospital at Snowlgobe 6, where they began to disintegrate, causing everyone who breathed it in to become unconscious. When Maxwell O'Keefe and his assistant Curtis Rabley arrived at Snowglobe 7 the Gappa attacked them, capturing Maxwell and using him as a host. Rabley was able to escape, killing a Gappa with bullet fire as he went. The Gappa used their telepathic link to awaken the infected people at the hospital and commanded them to come to Snowglobe 7 and hand themselves over to be eaten.
The Tenth Doctor and Twelve arrived at the Gappa's lair, distracting them and resulting in the telepathic link being broken. The Gappa pursued them but were defeated by guards using flamethrowers. The Gappa retained a telepathic link with Beth Cowley and made her take the "eggs" to the roof of the Rose Tower hotel. She accidentally fell, causing the dust to be spread and 2,000 people to be infected. The Doctor accessed Snowglobe 7 with an ambulance and detonated the engine from the old Modrakanian's ship. This killed the Gappa and blew up Snowglobe 7. (PROSE: Snowglobe 7)
When the Tenth Doctor and Donna were in what they believed to be an Antarctica base, the Doctor believed that the creature menacing the humans was a Gappa, but the whole scenario turned out to be a lure created by Sebastiene. (PROSE: The Doctor Trap)