A PN sequence can be generated by a Finite State Machine (FSM).
You can use the block gr_trellis_encoder and feed it with 0’s at the
input in order to generate the PN sequence.
The only think you need to provide to it is a description of the FSM
(at this point we do not have a constructor that takes directly the
feedback polynomial and constructs the next state and output tables).
See the trellis documentation for details…
PS: now i see that we can generalize the trellis_encoder block to be
used also as a block with 0 inputs, ie, emulating an all-0 input