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The quick verdict
The bottom line: The GA-B2100 is the CasiOak perfected. It keeps the famous octagonal bezel everyone loves, then fixes the original GA-2100’s two weaknesses: it adds solar charging so you never change a battery, and Bluetooth for automatic time sync. If you are buying a CasiOak in 2026, this is the one.
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Introduction
The GA-2100 “CasiOak” became a phenomenon because it looked like a luxury octagonal sports watch at a Casio price. The GA-B2100 keeps everything people loved about that design and upgrades the insides: Tough Solar charging and Bluetooth connectivity. I reviewed it in five colors on the channel, and also compared it directly against the original GA-2100 – this article covers both videos.
Watch: the full video review
My GA-B2100 review – solar, Bluetooth, and all five colors:
And the head-to-head comparison with the original GA-2100:
Price and where to buy
The GA-B2100 typically sells for around $150 to $180. That is a step up from the original GA-2100, and the premium buys you solar and Bluetooth – both genuinely useful, neither a gimmick. The CasiOak’s popularity means fakes circulate, so buy from a reputable seller and run the dial-printing and case-back checks from my fake-spotting guide.
Design: the octagonal icon
The CasiOak nickname exists for a reason – the octagonal bezel echoes the most famous luxury sports watch design in the world, at roughly one-fiftieth the price. The GA-B2100 keeps the formula: slim 11.9 mm profile, 48.5 mm case width, and Casio’s Carbon Core Guard structure that keeps the weight down around 52 g.
It wears remarkably well for its width. The slim profile and short lugs let it slide under cuffs, and the light weight means it never feels like a burden. This is the G-Shock you can wear with a button-down shirt without looking like you are wearing hockey equipment – a trick the Rangeman GW-9400 cannot pull off.
Five colors were available at launch – black, white, red, blue, and green. The black is the stealth pick, the colored dials are the fun ones. All of them share the same dial layout: analog hands over a digital display, with the octagonal bezel framing everything.
Tough Solar: the upgrade that matters most
The original GA-2100 ran on a standard battery. The GA-B2100’s dial doubles as a solar panel, trickle-charging the internal cell from daylight and indoor light. In normal use you will never change a battery – and as I noted in my GW-M5610U review, skipping battery changes also means never compromising the water-resistance seal to open the case.
For a watch designed to be worn daily for years, solar is the single most practical upgrade Casio could have made.
Bluetooth: automatic time, easy settings
Pair the GA-B2100 with Casio’s phone app and it syncs time automatically – including daylight saving changes and world-time zone updates when you travel. No atomic signal needed, no manual adjustment after flights.
The app also makes setup painless. Setting alarms, world time cities, and the countdown timer through the phone beats button-mashing on the watch itself. Like the GBD-200‘s app experience, it is functional rather than beautiful – but you will use it a handful of times a year, not daily, so polish matters less here.
Bluetooth is low-energy and the solar cell covers the draw. You do not trade battery anxiety for connectivity – there is nothing to charge, ever.
Display and legibility
The analog-digital layout divides opinion. The hands give you at-a-glance time; the digital window handles date, world time, and stopwatch data. In daylight it all reads well. At night, the LED backlight illuminates the dial – though as with most analog-digital Casios, the hands can occasionally sit over the digital readout. It is a minor quirk you stop noticing after a week.
If you want maximum legibility above all else, the full-digital GW-M5610U remains the readability champion.
Toughness and water resistance
Carbon Core Guard plus 200 m water resistance: this is a real G-Shock underneath the pretty bezel. Swim, shower, knock it off a desk – it handles all of it. The slim case does not mean a fragile case.
GA-B2100 vs GA-2100: which CasiOak should you buy?
The comparison video answers this directly, but the short version:
- GA-2100 (original): cheaper, battery-powered, no Bluetooth. Still a great watch if the price gap is large.
- GA-B2100: solar charging, Bluetooth auto time sync, otherwise the same beloved design.
Unless the original is dramatically cheaper, the B2100 is the buy. Solar alone justifies the difference over a decade of ownership – no batteries to buy, no case openings, no time adjustments after travel. The original made the CasiOak famous; the B2100 makes it effortless.
What I do not like
- The analog hands occasionally cover the digital display – a known analog-digital quirk.
- The phone app is useful but unpolished.
- At this price you are approaching full-size G-Shock Mudmaster territory on sale.
- Resin case and strap at $150+ still feel like a lot for plastic, even if the engineering justifies it.
- No atomic Multi-Band 6 – time sync depends on the phone connection.
Specs
| Spec | Detail |
|---|---|
| Case size | Approx. 48.5 x 45.4 x 11.9 mm |
| Weight | Approx. 52 g |
| Case / bezel | Resin, Carbon Core Guard, octagonal bezel |
| Power | Tough Solar rechargeable |
| Connectivity | Bluetooth Low Energy, phone app |
| Water resistance | 200 m |
| Display | Analog hands + digital, LED backlight |
| Functions | World time, stopwatch, countdown timer, alarms, auto calendar, phone finder |
| Colors | Black, white, red, blue, green |
Pros and cons
Pros:
- The iconic CasiOak octagonal design
- Solar charging – no battery changes
- Bluetooth auto time sync and easy app setup
- Slim 11.9 mm profile wears well, even with cuffs
- Lightweight Carbon Core Guard build
- 200 m water resistance
Cons:
- Hands can cover the digital readout
- Companion app is functional, not polished
- Pricier than the original GA-2100
- No atomic timekeeping without the phone
- Resin build at a mid-range price
FAQ
Is the GA-B2100 worth it over the GA-2100?
In most cases, yes. Solar charging and Bluetooth auto-sync fix the original’s two real weaknesses, and the price gap is modest. Only buy the original if you find it significantly cheaper.
Does the GA-B2100 need battery replacements?
No. Tough Solar keeps the rechargeable cell topped up from light exposure. With normal wear you will never open the case.
How does the Bluetooth time sync work?
Pair once with Casio’s app and the watch corrects its time from your phone automatically, including time zones when you travel and daylight saving changes. It uses Bluetooth Low Energy, so the power draw is minimal and solar covers it.
Is the CasiOak too big for small wrists?
At 48.5 mm wide it is a substantial watch, but the slim 11.9 mm thickness and short lugs help it wear smaller than the numbers suggest. Small wrists should try it on – the GW-M5610U is the safer compact pick.
GA-B2100 vs GW-M5610U – which should I buy?
The M5610U is the practical pick: cheaper, atomic timekeeping, unbeatable legibility. The GA-B2100 is the style pick: the octagonal design, analog hands, Bluetooth. Buy the square for function, the CasiOak for form – both are solar and both are excellent.
What colors does the GA-B2100 come in?
Black, white, red, blue, and green were the launch colors. The black is the most versatile; the colors are more fun.
Final verdict
The GA-B2100 is what the CasiOak should have been from the start. Same head-turning octagonal design, now with solar power and Bluetooth time sync handling everything in the background. It is the rare upgrade that changes nothing you loved and fixes everything you tolerated. If you want one stylish G-Shock that does it all, this is it.