The M series cameras are simple and robust cameras, the most famous being the MX and ME Super. These are classics of the era.
The MX was considered a pro camera with its depth of field preview, full mechanical construction and interchangeable screens). The ME super was considered more convenient consumer model with it’s aperture priority function.
The MG seems to be a lower spec version of the ME Super. It does offer aperture priority but shutter speed only run to 1/1000th of a sec. Like the ME Super its viewfinder coverage (92%) is much lower than the MX (95%).
This wouldn’t be regarded as a pro camera and I would guess that it is amongst the cheaper of the M models. However, I have personally owned an ME Super and I know that these are excellent cameras.