Posting TinyGPS++ co-ordinates as paramaters with GPRS
One solution may be to split the commands:

gsm.print("latitude=");
gsm.println(latitude);


I'm not sure how you should pass multiple parameters; if it is the usual syntax you can write something like

gsm.print("latitude=");
gsm.print(latitude);
gsm.print("&amp;longitude=");
gsm.println("longitude);


If I have the time I'll research into this a bit.

To check what is the problem, you may try to send "static" messages, like

gsm.println("latitude=12.345");


To check if this format is the correct one

EDIT: after a very quick research

It looks like the AT commands are a bit more complicated. In order to send the parameters, at least on a SIM800L, you should:


send the HTTPDATA command (the first parameter is the length of the payload)
wait for the board to send you the command DOWNLOAD
Send the body of the request, which is formatted as a JSON object. This means you have to send something like { "latitude" : 12.345, "longitude": 98.765 }


After all, it is much faster to use a library already developed for this, for instance ArduinoSIM800L (check a library available for your GPRS modem, or modify this one)