DB:NO RESP Using inet.httpPOST on loop()
Like I told you before, you have 2 instances of SoftwareSerial active, but you're allowed to listen to only one at a time (implementation rules). So you have to alternate between calling listen() on each instance; any data arriving at any port while its not the active port will be discarded. inet.httpPOST() worked in setup() because you called the function before begin()ing serialGps. Calling serialGps.begin() immediately made serialGps the active port, severing communication with the modem. 


Add this line to SIM900.h in the library, under the public methods:

void listen();

Add this method to SIM900.cpp:

void SIMCOM900::listen(){
   _cell.listen();
}

Save the files and add gsm.listen() right before gsm.SimpleWriteln("AT+CIFSR"); in loop().
Re-initialize GPRS, like you did in setup() with attachGprs(), right after gsm.listen() in loop().
Make serialGps.listen() the last  statement in loop() immediately after the inet.httpPOST() call. Use braces to enclose the body of the while loop.


Hopefully this should solve your problem.