Adding a Geofence on a Nodemcu gps tracker

hello, blynkers. I’m a newbie to the blynk and IoT world and I have been tasked to create a GPS tracker with a geofence capability. I’m using a GPS module Ublox Neo 6M and NodeMcu ESP8266 and Blynk for my tracking. The tracking part works fine but now i want to add Geofence. Help me out

Thank you in advance