Simone,
please take a look at the extensive documentation in:
gnuradio_trunk/gr-trellis/doc/gr-trellis.html
You need to first create an FSM object that describes your finite state
machine (using the generator polynomials you have)
and then use this as an argument to the Viterbi decoder.
The reason for this is that the Viterbi decoder is not only used for
decoding convolutional codes, but for numerous other applications; thus
the need to relate it only to the underline finite state machine…
Please take a look at the numerous examples given and let me know if you
have any further questions.
Achilleas