Dear Klaus,
I am working on a project involving button based action and later on GPS, more or less what are you looking for but I will have a LED Matrix attached also.
Here is my post for the LED Matrix http://www.seeedstudio.com/forum/viewtopic.php?f=71&t=6793&start=10 ,
use the last schematics on the 2’nd page and make abstraction of the LED Matrix wiring. Wire only the button on the
Breakout board (theoretically you can eliminate the Breakout and use the INTERRUPT pins on the RePhone GSM+BLE directly but due to my lack of codding skills someone advised me no to do it so I use the Breakout for button control).
Also I posted the code and there you will see how it is deal with button presses to change the matrix display; in your case will be to adapt the code to send the GPS coord. when a button press is detected.
The DOG Collar project it is great value since it shows how to interact with the GPS and the RePhone, I am reading it again and again and try to understand every step of that code.
Hope you can post your code so others can have a look and learn from …, probably your bigger problem like mine will be to come up with the code. Do not worry, I see the RePhone forum getting bigger and people helping others 
(Thanks! everyone for that)
Nice application based on real life need you identified! good luck
Catalin
PS. are you with Arduino IDE? if yes when you access a pin you need to address it as a number only not as
pin D3 but as pin 3 for example