What specific version of LAMMPS do you have installed?
I cannot test this specific version on my test machine since I have currently a different MPI version installed and am making tests with that (LAMMPS Windows Installer Packages with support for MS-MPI - Testers needed).
But for the non-MPI version I see the following:
C:\Users\akohl>lammps-shell
LAMMPS Shell version 1.2 OS: Windows 11 21H2, Windows ABI 6.2 (9200) on x86_64
MinGW-w64 64bit 9.0 / GNU C++ 11.2.1 20210728 (Fedora MinGW 11.2.1-3.fc35) with OpenMP 4.5
LAMMPS (23 Jun 2022 - Update 2)
using 2 OpenMP thread(s) per MPI task
Loaded 1 plugins from C:\Users\akohl\AppData\Local\LAMMPS 64-bit 23Jun2022\plugins
Loaded 1 plugins from C:\Users\akohl\AppData\Local\LAMMPS USER-VCSGC Plugin 23Jun2022
LAMMPS Shell> plugin list
Currently loaded plugins
1: command style plugin hello
2: fix style plugin sgcmc
LAMMPS Shell>
Which confirms the plugin is loaded by default and the sgcmc fix is available.
Of course, this will only happen when the LAMMPS version and the plugin version match exactly.