Generate supercell from unit cell

GROMACS version:
GROMACS modification: Yes/No

Greetings,

I have a unit cell structure in gro file format. This is a hydrate unit cell that contains water (host) and gas (guest) molecules. I want to replicate this unit cell and generate a supercell, but I do not want to replicate exactly the same unit cell. Basically, I want to combine unit cells that have different numbers of guest molecules so I can obtain the desired guest/host ratio for the supercell. Do you know how can I achieve this in GROMACS?

Thanks!