Hi
this old post may be useful
The option is not support in the current version of GROMACS.
The feature for embedding a protein in a membrane will be retained, but probably in a different form, such as gmx membed (see release note https://manual.gromacs.org/current/release-notes/2021/major/deprecated-functionality.html#gmx-mdrun-membed)
An alternative to embed a protein in a lipid bilayer is CHARMM-GUI.
Best regards
Alessandra