You can now pass additional arguments to gdbus-codegen using the extra_args keyword. This is the same for the other gnome function calls.
Meson now automatically adds autocleanup support to the generated code. This can be modified by setting the autocleanup keyword.
For example:
sources += gnome.gdbus_codegen('com.mesonbuild.Test',
'com.mesonbuild.Test.xml',
autocleanup : 'none',
extra_args : ['--pragma-once'])