| Name | Source | Notes |
| Gabri Rates | Vampire Idol | Vampire-humanoids |
| Gadmeer | Stargate SG-1 | |
| Gauna | Knights of Sidonia | A race of Shapeshifting Eldritch Aliens |
| Gaim | Babylon 5 | |
| Galactoids | Superjail! | A race of party creatures. |
| Galactus | Marvel Comics | |
| Galaxoid | Calvin and Hobbes | |
| Gallaxhar | Monsters vs. Aliens | Believed to be a space squid, since he stated he was "but a squidling" when he was young. |
| Galvan | Ben 10 | Small, frog-like humanoids from Galvan Prime, Galvans are exceptionally intelligent, especially in inventing and innovation. |
| Galvanic Mechamorph | Ben 10 | Made of a liquid-metal like nanotechnology, Galvanic Mechamorphs can fuse with technology and increase its technical capabilities, and can replicate any technology they have previously augmented with. They live on Galvan Prime's moon, Galvan B, and were created due to a Galvan terraforming experiment. |
| Gamilons/Gamilus | Space Battleship Yamato/Star Blazers | Humanoid |
| Gamorrean | Star Wars | Rotund, pig-like humanoids |
| Gand | Star Wars | |
| Ganymean | James P. Hogan's Giants Series | Large technologically advanced humanoids |
| Gaoul | Titan A.E. | Leathery-winged, pterosaur-like aliens. Native to Sesharrim. |
| Garthlings | David Brin's Uplift Universe | |
| Garys | Adventure Time | an alien species |
| Gashlai | Twilight Imperium | Organized as "Embers of Muaat"; burning beings whose metabolism is based on a unique energy-to-mass conversion |
| The Gelth | Doctor Who | Gaseous humanoids, they can either exist in gaseous mediums or inside the corpses of others. |
| Gedd | K. A. Applegate's Animorphs | |
| GELF | Red Dwarf | Genetically Enhanced Life Form — a wide variety of artificial 'aliens' created by human scientists. Some are humanoid, others are not. All life seen in the Red Dwarf universe originated on Earth, there are no true aliens of extraterrestrial origins. |
| Gelgameks | South Park | Toad-like with slimy green skin and large eyes. Females are claimed to have vaginas that are three feet wide and filled with razor-sharp teeth. Gelgameks also represent the Roman Catholic Church. |
| Gemini | Battlelords of the 23rd Century | |
| Gems | Steven Universe | Sentient rocks that project a human-like form. Gems are not sexually dimorphic but are feminine in appearance and are addressed as women. As shown in the "How Are Gems Made?" short, they absorb nutrients from the ground and emerge from the planet as a fully formed Gem. |
| Geochelone Aerios | Ben 10 | Peaceful, philosophic turtle-like creatures from the planet Aldabra in the Andromeda Galaxy, Geochelone Aerios can unleash powerful torrents of wind from the holes in their torso and control air in other ways, and they are immune to magic and attacks based on mana, or life energy. |
| Geonosian | Star Wars | Insectoid humanoids |
| Geth | Mass Effect | Artificially intelligent robots created by the quarians, geth and other races | |
| Ghamans | Perry Rhodan | |
| Ghosts | Nexus: The Jupiter Incident | |
| Gibis | Les Shadoks | |
| Gigan | Godzilla films | Cyborg kaiju with claws for hands, a buzzsaw in his abdomen, a single red eye |
| Giygas | EarthBound | |
| Gigglepies | The Fairly OddParents | Rabbit-like aliens, all of them having floppy ears, a small heart-shaped nose, large shiny eyes, and a round head. They spread across entire planets by posing as the "Special Surprise Inside" a box of "Invader O's" cereal. After they enslave a planet, sell their merchandise, and suck it dry, they blow it up and move onto another planet. |
| Gill Men | X-COM: Terror from the Deep | |
| G'keks | David Brin's Uplift Universe | |
| Gladifers | | |
| Glapum'tians | Valérian and Laureline | |
| Glendaliens | Bravest Warriors | A species of slime-like creatures. After The Great Time Catastrophe, the Glendaliens created a machine to keep time running. After the machine broke down and began looping time, it is unknown what happened to the Glendaliens on the Moon of Glendale. |
| Gliscians | SCP Foundation
| Also known as SCP-1342-3. A species of tri-radially symmetrical beings from Gliese 445-C, a fictitious planet of Gliese 445. By the year 42,412, Glisians reverse-engineered radio technology from Earth's signals and contacted humanity, and shared technological advancements until humans devastated the Gliscian homeworld after the latter's invention of space-time technology. The dying Gliscians recovered Voyager 1 and its contents, from which they were inspired to create their own space probe and record | |
| G.R.A.I.S.E. | Melonpool | |
| Grans | Star Wars | Humanoid |
| The Graske | Doctor Who and The Sarah Jane Adventures | Small Humanoid |
| Great Race of Yith | H. P. Lovecraft | |
| Grendarl | Master of Orion III | |
| Grendlers | Earth 2 | Intelligent bipedal species known for their scavenging, hoarding, and trading. Though not a violently aggressive species they are very fond of human blood. |
| Gretchin/Grotz | Warhammer 40,000 | Humanoid |
| Grey alien | popular mythology | |
| Grob Gob Glob Grod | Adventure Time | a four-headed deity from Mars, and the brother of Magic Man |
| Grogs | Larry Niven's Known Space | |
| Grudeks | Farscape | Humanoid |
| Grue | Pitch Black | Humanoid |
| Grunds | Marvel Comics | Humanoid |
| Grundos | Neopets | Cute Alien who lived on the moon of Neopia. |
| Grunts | Halo | Short creatures, mostly used as fodder for bigger creatures in Covenant. Breath a gas native to their homeworld. |
| Guard | Alienoid | Humanoid |
| Guardians of the Universe | DC Comics | Small blue humanoids who are among the oldest life forms in the universe, originating on the planet Maltus before moving to Oa in the center of the universe. They are the leaders of the Green Lantern Corps. |
| Gubru | David Brin's Uplift Universe | |
| Gungans | Star Wars | An aquatic adapted species. They are masters of underwater combat, are fairly fast swimmers, and are proud warriors. |