Hi,
If you have Windows OS, you can use HyperTerminal or any other Serial Terminal Program to send command to GM862 via Serial Port. (you should use this method to ensure your modem, power, sim card and serial cable is OKAY)
I have no idea about UBuntu, you should search UBuntu forum for serial interface programs and methods. Look for Serial Emulator.
This is the reason you are not getting reply here.
Always interested to learn new thing, if you happened to find solution for UBuntu please share here.
M.Pathma