mGear can definitely rig asymmetrical characters but you need to think further beyond mGear. Maya does not handle mirroring accurately skinning (or any deformer weight for that matter) when an asset is not symmetrical and definitely not with asymmetrical geometry. I strongly recommend that if you are going for a fully asymmetrical character to proceed in the following matter.
- Have the model been posed symmetrically (T-pose or A-pose, both work even if both implies different results) and this mesh should be symmetrical. (This can be used as a technical geometry)
- Have the final character with asymmetry mesh and topology model in the same pose as the technical
- Rig the symmetrical one and transfer the rig to the asymmetrical one or wrap the asymmetrical to the symmetrical one.
This is of course a super fast recap of what you can do but there are other different ways to inject asymmetry into character assets. You can have your character with different topology on both sides but the pose is symmetrical, and you can inject into the character more asymmetry details by adding those changes into a blendshape target rather than the model itself… Or the asymmetry can be done with the rig itself (in some cases) depending on the mechanics that you are looking for. It is hard to give you an accurate response to your case without at least an image of your asset.
But for what concerns mGear itself, shifter will be able to build your rig if you pose the guides with asymmetry.
Hope this helps.