I use the Fossil Collider HR with Gadgetbridge and it's pretty great. Three are some downsides though:

• The set-up is a little bit of a hassle. You need to get the Collider's secret key, and that requires installing a jailbroken version of the Fossil app
• Any changes you want to make to the watch display have to be made in the Fossil app, which means you have to install that app and do a bit of a runaround whenever you want to change something

But aside from that, it's been a good year with my Collider HR and Gadgetbridge