G1, G01, G.I, G-1, or G One may refer to:
Science and technology
- G1 phase, in the cellular cycle
- G1 regulatory sequence for the insulin gene
- ATC code G01 Gynecological antiinfectives and antiseptics, a subgroup of the Anatomical Therapeutic Chemical Classification System
- Alkali metal, Group 1 of the periodic table
- G1, an informal group of fossil bird eggs from the Gobi desert that were later named Gobioolithus minor
Astronomy
- Globular cluster Mayall II
- YGKOW G1, a giant elliptical galaxy that lensed the Twin Quasar
- G1 star, a subclass of G-class stars
- G1, a cloud that passed by Sagittarius A* around 2001-2002
- g1, temporary designation of minor planet 2014 PN70, a New Horizons flyby candidate
Computing
- Garbage-first collector, of the Oracle HotSpot Java virtual machine
- G1, in ISO 2022 character sets, one of 4 groups of characters that may be swapped into the encoding space via control codes
Cameras
- Contax G1, a Rangefinder camera made by Contax
- DSC-G1, a 2007 Sony Cyber-shot G series camera model
- PowerShot G1, a Canon camera
- Panasonic Lumix DMC-G1, a digital camera made by Panasonic
Military
- Ordnance QF 25 pounder (South African Army designation: G1)
- G-1 (submarine), a class of early-20th century submarines constructed by the Lake Torpedo Boat Company of Bridgeport, Connecticut and purchased by the US Navy
- Char G1, a French tank
- G1, the designation used by the West German and Turkish armies for the FN FAL rifle
- G.I, a German designation for several World War I heavy bombers:
- AEG G.I
- Albatros G.I, a German aircraft whose development was the Albatros G.II 1916 aircraft
- Friedrichshafen G.I
- Gotha G.I
- Hansa-Brandenburg G.I, a 1916 bomber aircraft
- Fokker G.I, a 1937 Dutch heavy twin-engined fighter plane
- G-1 military flight jacket, a type of flight jacket
- Kampfgeschwader 55 (historic Geschwaderkennung code G1), with the Luftwaffe in World War II
Sports
- G1 Climax, an annual professional wrestling singles tournament held by New Japan Pro Wrestling
- Group One, the highest level races in horseracing
Transportation
- G1 Aviation G1, French ultralight aircraft
- G1 Beijing–Harbin Expressway, in China
- Alfa Romeo G1, a 1921 Italian car
- Gorkha Airlines (IATA airline designator)
- One of the G-series Toronto subway cars
- SEPTA Route 15, All Stops service pattern labelled G1
Other uses
- G1 (website), a Brazilian news portal owned by Grupo Globo
- G1 Group, a Scottish hospitality and leisure operator
- G1 licence, the first tier in the graduated licensing programme for Ontario's new drivers
- G.One (character), an android super hero
- Gangwon No.1 Broadcasting, a Seoul Broadcasting System television channel in Gangwon Province
- Godzilla Minus One, a 2023 kaiju film often abbreviated to G–1
- T-Mobile G1, the first publicly available Android smartphone
- Transformers: Generation 1, a toy line which ran from 1984 to 1992
- The Transformers (TV series), an animated television series meant to promote the toy line (popularly referred to as Generation 1)