Gadgets and apps for elite sprinters?

I am afraid that the accuracy of the GPS (even in combination with accelerometer and gyroscope) is way too insufficient for measuring 100m (or even 400m) sprints up to 1 hundredth of a second, which is likely needed on the elite professional level. You would still need sensors in the starting blocks (or the starting gun), and on the finish line for getting accurate times.

If you do already have some sensors, and the sensors are able to broadcast the data over  ANT+ or over Bluetooth, then you could hire a Connect IQ developer for adding the transfer and the processing of the data from those sensors to the watch. The problem though is, that the range of ANT+ is just 5-10m, and Bluetooth range is around 10m, so a reliable communication between the sensors and the watch may be challenging.

You could also perhaps look at GPS vests, such as from STATSports.com or from Catapult.com - they are more designed for professional athletes. I am still not sure whether they are well suitable for sprinters, since AFAIK they were developed mainly for team sports like the football (soccer), but it may be worth of testing.

• APEX Athlete Series | Features (statsports.com)
• Athlete Monitoring System | Athlete Software | Catapult