If you want reliable tool then I would suggest to use Migrate Mac OLM to Gmail Converter. This software provides option to import OLM files into Gmail account in a simple manner. User can use the batch conversion function to convert a single file and a batch file. User can save data and export attachments and emails in various file formats like EML, EMLX, Zimbra, MSG, HTML, etc.
Step 1: Configure your Gmail account for IMAP.
Step 2 Add IMAP-enabled Gmail account into Outlook for Mac.
Step 3: Transfer OLM File Data to Gmail.
Step 1: Export OLM Contacts to .
Step 2: Convert txt file to Comma Separated Value (CSV) File Format.