I've read your question carefully and I understand what kind of solution you need for creating an OLM to Gmail. This software provides a Message Filtering option that allows users to select OLM items between Start Date and End Date. With the OLM to Gmail migration, you can check the preview of the mailbox data items and attachment before the actual conversion takes place.
If you are looking for the best Mac OLM to Gmail Converter then, you can go with CubexSoft OLM to Gmail Tool which allows users to successfully import Mac OLM emails to Gmail account including all emails, notes, tasks, calendars and contacts. This application is very useful for users to migrate Mac OLM to Gmail in bulk at once without losing any data. One can easily download its free demo version on any Windows OS like Windows 10, 8.1, 8, 7 etc.