You can easily import Mac OLM file to Gmail using Third-party software like KDETools Mac OLM Converter. This software support ‘Batch Conversion’ option to perform bulk conversion of OLM items to Gmail account. There is no risk of data loss during the conversion process. You can also download the demo version first and migrate first 30 emails from each mailbox.https://www.kdetools.com/mac/olm/gmail/