You probably should work on getting the network code functioning on it's own, before putting it into a thread. Some tricks are to wrap the network code in an eval with an alarm. See Thread Safe alarms?.

You are not showing a complete code for this, so as a guess, one of the other mistakes may be that you are declaring your GSM network module globally in the main thread.... try to confine all network code, including the use statement, to the worker thread. Many modules are still not thread safe.