When will g5 instances be available for batch transform?

The G5 instances should be available soon for SageMaker transform as well. In the meantime, you could use SageMaker Training Jobs with custom container to do inference as well. Then you can retrieve the predictions results after unzipping the default output file model.tar.gz which is provided by the SM Training Job. Another alternative is to use the G4DN GPU instances with your SageMaker Batch Transform.