How to send data from Arduino Uno with SIM900 GPRS to Firebase
Like you said, Firebase only accepts HTTPS traffic, which the Arduino Uno cant generate. Your HTTP requests first need to be sent through a proxy server that supports SSL, before being sent on to your Firebase database using a PUT or POST request. This site explains how to do everything from setting up your own proxy server to writing the GPS data (in your case) to Firebase. You may want to look at alternatives if this is too tedious for your purposes.