I’m not sure if it’s the done thing to advertise offered services here (as oppose to request a needed service) but here goes.
I have been working on an open source Google Group to Discourse migration script which I have now successfully used to migrate several Google Groups. Other people report they have successfully used the script too, and it’s there for anyone to use.
If, however, you would like your Google Group exported to Discourse for you, I can provide this service. I can also set up and manage your Discourse instance, if you would like to ‘outsource’ that work. Rates are negotiable and depend on the size and complexity of the migration, and any specific requirements your destination Discourse instance.
PM me for more information