As you are looking for the best software to import Mac OLM to Gmail account then in my opinion, you can use KDETools OLM to Gmail Importer. User can use the batch conversion function to convert a single file and a batch file. This software provides so many advance and unique feature "split option" so that users can easily split the oversized files into small files with this feature.https://www.kdetools.com/mac/olm/gmail/