G-Shock GA-2100 “CasiOak”: The Thin Octagonal G-Shock

The G-Shock GA-2100 is the most important G-Shock of the past decade. Launched in 2019 at US$99, it combined an unusually thin case with an octagonal bezel that collectors immediately nicknamed the "CasiOak" for its resemblance to the Audemars Piguet Royal Oak, and it sold out repeatedly.

The Carbon Core Guard

The GA-2100's key innovation is the Carbon Core Guard : an inner case of carbon-fibre-reinforced resin that protects the module and carries the shock-resistant structure, allowing the outer case to be dramatically slimmer. The result measures 48.5 × 45.4 × 11.8mm and weighs just 51 grams, making it the thinnest G-Shock of its time while retaining full shock resistance and 200 metres of water resistance.

Design and Functions

The octagonal bezel and band shapes deliberately recall the original 1983 DW-5000C, giving the watch a retro G-Shock character despite its modern construction. The analogue-digital module 5611 provides world time across 31 time zones, a 1/100-second stopwatch, a countdown timer, five daily alarms and a double LED light, powered by batteries rated for about three years.

GM-2100 and GM-B2100

The CasiOak's success prompted upmarket versions. The GM-2100 of 2021 added a stainless steel bezel over the resin case at around US$220. In 2022 the GM-B2100 went fully metal, with a stainless steel case, bezel and bracelet, a screw-down caseback, Tough Solar charging and Bluetooth phone linking, at US$550–$600 depending on finish.

Why It Matters

The GA-2100 showed that a G-Shock could be slim, stylish and wearable with almost anything, without giving up toughness. It brought a new audience to Casio and made the "CasiOak" one of the most-modified watches in the hobby, with an entire aftermarket of custom cases and bezels. For other value picks, see the best watches under $500.

Frequently Asked Questions

Why is the GA-2100 called the CasiOak?
Its octagonal bezel resembles the Audemars Piguet Royal Oak, so enthusiasts combined 'Casio' and 'Royal Oak' into 'CasiOak'.

How thin is the G-Shock GA-2100?
11.8mm, with a 48.5 x 45.4mm case and a weight of 51 grams.

What is the Carbon Core Guard?
A carbon-fibre-reinforced resin inner case that protects the module, allowing a thinner outer case without losing shock resistance.

What is the difference between the GA-2100, GM-2100 and GM-B2100?
The GA-2100 (2019) is resin; the GM-2100 (2021) adds a steel bezel; the GM-B2100 (2022) is full metal with Tough Solar and Bluetooth.

Related Reading

• Casio: Brand History
• Casio G-Shock
• Audemars Piguet Royal Oak