My GW60 battery is 50% capacity, so have investigated battery replacement options.
I've sourced a supplier from China that offers a LP572225 package with a capacity of 310mAh (Nb/ OEM battery is 250mAh). Min quality is 5 pieces. These batteries have a fixed shelf life, so they need to be fitted. I don't do repairs in summertime. So, I'm finding out if there are others who want their batteries replaced. The cost of repair works out to $60 a watch. I want to repair my watch first. If I'm successful, then I'll accept requests.
There's some risks: a) This is a larger capacity battery. If the GW60 designers follow the charging regime in the datasheet, then we're golden. If not, then watch will probably stop working after a while due to battery damage. We will never know until I try it. b) Dealing with the Chinese. What could possibly go wrong ?
I cracked open my watch tonight. This battery will fit.
I won't buy 5 batteries to fix 1 watch if no interest - hence this canvassing.
Cheers J
Good thinking jn1, the motion has taken over from my watch, so I'll pass on that. But wish you luck, It'll be interesting to hear how you go.
Not really relevant, but my Samsung Gear S3 battery was only lasting half a day. I bought a new battery (with free special screwdriver) from some dodgy Chinese guy on ebay. The fitting was difficult but amazingly, battery life is back to two days.
I think I can confidently post this now. GW60 has been running flawlessly for a week. The new battery has the same rated capacity as old battery, but seems to have less capacity. There again, it is an eBay jobbie.
Ordering
The battery order was from eBay. This was cheaper than the one mentioned above, but have no idea the quality and conformance to the claimed details they provided on the ad. With the order, you get 2 x 602025 250mAh LIPO batteries. The cost was $30AUD.
"3.7V 250mAh Rechargeable lipo Battery For headset MP3 MP4 GPS Smart band"
www.ebay.com.au/itm/183400892805
The repair
It was a fiddly job with the straps getting in way and I removed the vibrator as I don't use this feature; it got in the way too.
I took printed circuit board (PCB) out to solder battery terminals. I found it too tight to do with it inside the watch enclosure (soldering iron got too close to the power supply SMDs).
If I was to do this job again, I would keep the original battery terminals (ie: cut battery leads off and reuse them), because the switch brackets were activating watch when battery was connected and it saves reflowing battery terminals on the PCB (a risky procedure).
Nb/ PCB has no problems being power isolated (ie: cutting old battery off and leaving it off). Also, don't remove the shield mat on the back of the Antenna (the gold square thing in the left/middle picture below). You'll make the watch less sensitive (ie: raise noise floor of receiver) than it already is.
The old and new batteries. The new battery is top/right in the pic below.
I stuck Kapton on the side of the battery facing the PCB to prevent any short outs. You can probably use masking tape (don't use electrical tape).
Nb/ Fun fact: The Apollo lunar landers were covered in Kapton. So, if you want some of this stuff, fly to the moon and rat it off the LMs 
The 602025 package size (6mm thick) was too fat for the watch enclosure. It was competing for space with the PCB. This was a show stopper.
So I had to dremel about 0.2mm off the back cover to make it fit. If you do this, make sure it's got space and then some for thermal expansion (ie: The battery is metal and will expand with heat). I fitted this battery in winter (8C). Our summers get +40C.
With the gasket, I took it out, gave it a clean and lubed with Silicon grease.
With everything fitted, I reflashed the firmware, just to be safe.
So far, I've used it a few times driving to work, riding push bike, riding motorcycle. It charges, it downloads, it logs okay. I done drop tests, heat testing where I left it on the dash board of car in the sun on a 20C day (it would've got 50C in the car I reckon). So, I'm happy. As I said at the start of this post, the battery seems to have slightly less capacity. I sailed on Saturday for 3.5hrs, and it had 1 bar left. I remember when my GW60 was brand new, I could sail for 5 hours and still have 1 bar.
I will monitor battery performance over summer. "Watch" this space 
Thanks JN, so the recommendation is to find a 55 instead of a 60 if you don't want the thickness hassle.
I've been told that lipo's capacity improves with the first few charges,
