I would like to suggest a third-party tool to migrate OLM to Gmail i.e. KDETools OLM to Gmail Migration Tool. This Application shows a free preview of the OLM mail with attachment. OLM to Gmail Migration tool operates with all the Windows Version. and OLM to Gmail Mailbox converter supports multiple file formats such as PST, EML, EMLX, PDF, HTML, MBOX, NSF and etc.