GliderLink is not really a replacement for Flarm. GliderLink can make you aware of traffic that you might not have seen otherwise, but it cannot give you warnings that you are on a collision course. This is because GliderLink only broadcasts your location every 20 seconds, a limitation imposed by the goTenna Mesh (5 transmissions per minute). If you don't have Flarm then GliderLink is certainly better than nothing, but I will be using both, personally. It would be cool if I could display Flarm traffic in GliderLink. I'll have to look into whether that is technologically feasible.

Regarding the frequency goTenna Mesh uses, their FAQ says:

goTenna Mesh will transmit at 1 W on 902-928 MHz in the United States, and will vary throughout other countries. For example: goTenna Mesh will transmit at 0.5 W on 869.4-869.65 MHz in Europe and other CE countries, and the device will auto-tune itself to the appropriate frequency band and power output based on the location where it's being used.

In my research on 902-928 MHz in the US, I could not find any specific prohibition against using the frequency while airborne. I asked goTenna if the Mesh was legal to use in the air, and they said they didn't know one way or the other. I did find that the original goTenna uses a MURS frequency that is not legal to use in the air, but MURS is not used by the goTenna Mesh. As far as Europe or other countries go, I haven't yet done any research on the legality of using the goTenna Mesh in the air. Anyone know?

Speaking of, I apologize to users outside the US for not having maps available right now. I believe I have a solution at least for Europe and I plan to add it to the app. I'd like to get confirmation on the legality of airborne use of the goTenna Mesh in Europe first though.

Regarding battery life, GliderLink is pretty easy on the battery from what I've seen. Putting your phone in airplane mode certainly helps conserve battery. Since GliderLink doesn't depend on the cellular network, you can do this and still use the app. The goTenna Mesh has it's own battery which should easily last all day with GliderLink. I've never drained the goTenna battery below about 50% if I recall correctly.

Regarding XCSoar and TopHat integration, it should be possible. I haven't talked to any of their developers, but I think it would be great if they wanted to integrate it.

One thing to keep in mind is that the goTenna Mesh, and GliderLink by proxy, requires Android 4.3+ and Bluetooth LE (4.0). Older devices like the Dell Streak don't meet either of these requirements, unfortunately. If you're wondering what Android phone to run GliderLink on, the Samsung Galaxy S series from S6 on have pretty bright OLED displays that are reasonably readable in sunlight.