Yes, you can use a Mashing/Grain bag while brewing with your G30 - its implementation will depend on whether you are wanting to use the Grain Basket as well.
If you are wanting to use the Grain Basket with your mashing bag, use the mashing bag on the outside of the Grain Basket as this will provide the best filtering results.
If you are not wanting to use the Grain Basket with your mashing bag, then you will want to ensure the grain bag does not touch the heating element while brewing - this will cause burning and you will end up with all your grain in the unit.
Additionally, your water calculations will likely need to be adjusted when using a mashing bag as the recoverable deadspace volume will change based on the grain bag used and how low in the unit it is placed.
You can do this with the other brewing units as well if you are able to find a bag as large as needed for the particular brewing unit.