Casio G-Shock GA-2100 "CasiOak" (ref. GA-2100-1A1)

The Casio G-Shock GA-2100-1A1 is an analog-digital quartz watch introduced in 2019 as the first Carbon Core Guard model with a retro octagonal bezel, nicknamed the "CasiOak" for its resemblance to the Audemars Piguet Royal Oak. The all-black "1A1" is one of the launch references, built on a carbon fiber-reinforced resin case measuring 48.5 x 45.4 x 11.8 mm and weighing about 51 grams, making it among the thinnest and lightest G-Shocks. It carries 200 m water resistance, module 5611, world time across 31 zones, a 1/100-second stopwatch, and a roughly three-year battery.

Casio launched the GA-2100 series in Japan in August 2019, debuting with three references: the black-and-grey GA-2100-1AJF, the blacked-out GA-2100-1A1JF, and the red GA-2100-4AJF. Japanese list price was 13,500 JPY plus tax, and Casio America released the line at $99 USD, a figure reported by Professional Watches at launch.

The GA-2100 was built around Casio's Carbon Core Guard structure, described by Professional Watches as a case "manufactured from resin and carbon composite, referred to as Carbon Core Guard, with a steel caseback." That construction let Casio compress the case to 11.8 mm; Professional Watches called it "the thinnest combination G-Shock model ever produced."

The watch quickly earned the nickname "CasiOak" for its octagonal bezel, which many observers likened to the Audemars Piguet Royal Oak. Teddy Baldassarre's write-up notes that Casio's design team frames the shape differently, saying "the primary inspiration was the bezel on the earliest G-Shock models — which was, upon close examination, technically an octagon, albeit a much more rectangularly oriented one," tracing the lineage back to the original DW-5000C.

The GA-2100-1A1 runs Casio's module 5611, a combination analog-digital quartz caliber. It drives three analog hands over a small LCD window, with G-Central listing a stated accuracy in line with a standard quartz movement and a battery of two SR726W cells rated for approximately three years of life.

Functionally the module is broad for the price. Teddy Baldassarre's spec listing records world time across 31 time zones (48 cities plus coordinated universal time) with daylight-saving and home-city swapping, a 1/100-second stopwatch, five daily alarms with an hourly time signal, a hand-shift feature that moves the hands clear of the digital display, and a full auto-calendar programmed to the year 2099. Rated accuracy is ±15 seconds per month.

Illumination is handled by what G-Central describes as a "Double LED" Super Illuminator system with a white light and a selectable 1.5- or 3-second afterglow, lighting both the LCD and the dial.

In his review for Watch Clicker, the reviewer Will lists the case at 45 mm wide, 11.8 mm thick, and 48 mm lug-to-lug with 200 m water resistance. He writes that on handling the watch "you will immediately notice how light it is," and that the carbon-fiber content makes it "even lighter on the wrist than most other G-Shocks available today."

Will characterizes the case as "slimmer and lighter than most G-Shocks available and is the thinnest G-Shock Casio currently sells," which he says gives it a "more subdued" presence than a typical G-Shock. He adds that "it is one of the few models the brand offers that can adapt to a variety of situations," noting its suitability across different wrist sizes and settings.

The slim profile is central to its reception: Professional Watches, in its launch coverage, framed the 11.8 mm case as "the thinnest combination G-Shock model ever produced," the trait most reviewers tie to its comfort.

The GA-2100-1A1 is the all-black member of the original 2019 trio, alongside the black-and-grey GA-2100-1AJF and the red GA-2100-4AJF, all sharing the same resin case and module 5611.

The platform has since expanded well beyond the base resin model. Teddy Baldassarre's CasiOak guide catalogues the GA-B2100 with Bluetooth and Tough Solar power (around $150), the GM-2100 with a metal-covered case and resin strap (about $200), the GMB-2100 with a full metal bracelet ($550–$600), and the smaller GMA-S2100 at 42.9 mm.

Across these generations the octagonal case profile and analog-digital layout are retained, with changes centered on materials (resin versus metal cladding) and power/connectivity (standard battery versus solar and Bluetooth), so the GA-2100 resin models remain the entry point to the family.

The GA-2100-1A1 launched at $99 USD in the United States, per Professional Watches' 2019 coverage. It remains a current catalogue model sold through Casio and authorized retailers; Teddy Baldassarre, an authorized G-Shock dealer, lists the GA2100-1A1 at a $110 retail price, reflecting the modest price increase since launch.

Buyers choosing within the family should weigh the base resin GA-2100 against the connected GA-B2100 (Bluetooth and Tough Solar) and the metal-clad GM-2100 and GMB-2100, which Teddy Baldassarre's CasiOak guide lists at roughly $150, $200, and $550–$600 respectively. The resin GA-2100 remains the least expensive way into the octagonal case.

Because the reference is widely produced and still cataloged, it is broadly available new from authorized sellers rather than being a limited or discontinued piece; the all-black 1A1 in particular is one of the most common configurations.