There's still a good deal more to this. You should do a comprehensive search for migration tools to do this right rather than trying to get it working with GAM.
Having said all that:
- To date, GAM has not added support for creating spaces in importMode.
- messages would then need to be created by GAM with fields like createTime set. To date that does not exist either.
So no, GAM really does not support Chat migrations at all. You can file a feature request to add GAM support for these basic Chat migrate API calls:
Jay