Please direct all LAMMPS questions to the LAMMPS mailing list.

As the fix gcmc documentation makes clear, it is intended primarily as a way to allow the number of (small) molecules in a system to be adjusted via random insertions and deletions. As an additional feature, it allows those molecules to be randomly translated and rotated. No where in the documentation is there any suggestion that these features would be useful for equilibrating a dense system of stiff polymer chains. That is because they would not.

Equilibrating polymer systems is an intrinsically difficult problem in molecular simulation for which there is no simple software solution. If you read the literature you will discover a variety of strategies, many of which can be executed using LAMMPS. There are also a variety of polymer builder tools that have been developed for this specific purpose.