Hi,
So you're basically using the iOS app as a User Interface to control your GPS Tracking Platform.
I've written a number of similar apps, for custom home automation and lab hardware.
These generally communicate with the iOS app using Bluetooth 4.0/LE, but other techniques are possible.
I develop apps for iPhone/iPod Touch/iPad running iOS 9, using Xcode and Swift.
You can see some examples of my work, and client reviews of my completed projects, on my Freelancer profile page.
I'd be interested to hear more about your project, including details of the hardware, the communication method, and the API.
If you'd like to reply to this message, we can discuss things further, before you make your decision on awarding the project.
Best Regards,
- Rob Dixon.