Since you are looking for the best software to import OLM to Gmail Account, I think you can use KDETools OLM in Gmail Importer. The user can use the OLM to Gmail importer application. It can just import the OLM data which will be imported directly into the Gmail account you choose with no errors.
Go with CubexSoft OLM File Converter Tool which allows to import OLM to Gmail and other formats such as Office 365, G Suite, Yahoo, IBM Verse, Zimbra, Lotus Notes, IMAP, WLM, PST, PDF, MSG, MBOX, HTML, CSV, etc. It easily moves complete OLM mailbox data to Gmail account including all emails, contacts, tasks, journals, notes, and tasks. This OLM to Gmail Tool allows users to export single or multiple Mac Outlook OLM files into Gmail at the same time without losing any data. One can also try its demo pack on any macOS & Win OS machine.
To import OLM to Gmail, try BitData Migration OLM Converter Tool. It import bulk or selective OLM files into Gmail account in single processing. This attractive application also convert Outlook for Mac emails to Gmail account including all emails, contacts, calendars, notes, etc. Moreover, it supports all latest and previous range of macOS, Win OS, and Outlook. Download its demo pack to export only 25 items for free of cost.