OK, so that is already covered.
Next question: is it actually causing a problem or is it just noise you are observing. The reason I ask is because your measuring equipment (oscilloscope I presume) will have probes that inject RF from the GSM source.
The other thing you can try is to add a small ceramic capacitor directly from the TSOP output to it's ground pin. This time you should use a lower value, maybe 100pF so it doesn't distort the output waveform. If you do the same at the other end of the track/wire, across the input of the following circuit it might also help.
Brian.