As others have stated, without providing any useful information, no useful help can be provided.
For example:
What brand and model of PLC? (Siemens, A-B, ABB, Mitsubishi, Rockwell, GE, etc...) S7, PLC5, ControlLogix, etc.
What brand model of GSM modem? Does it have serial port interface? Is it RS-232, RS485, or ??
All of the major brands of PLCs already have various radio link modules and products, so are you using one of them, or a 3rd party brand?
All of the major brand PLCs support several protocols that could be used over radio link (Profibus, DataHighway, DH485, Cnet, etc). What protocol(s) are the PLCs using in the application, and do you need to implement one of those over the link, or a more simplistic data interchange.
What have you tried to do so far?