1. Enable IMAP in Gmail

Sign in to your Gmail account.
Click on Setting icon and select Setting.
Select Forwarding and POP/IMAP tab.
Enable IMAP Access and click on the Save Changes button.
Create a label to save the Mac OLM data separately.

2. Configuring Gmail Account in Mac Outlook

Open Mac Outlook. Go to the File menu and select Accounts.
Click on Emails option.
Enter your Gmail account credentials.
Click on Add Account button. Now, Gmail account is configured with Outlook for Mac.

3. Import OLM File to Gmail Account

Open Outlook for Mac on your machine and select the folder from which you want to move OLM emails.
Right-click on that folder and select Move then select Copy to folder option.
A dialog box will pop up, here select the newly created label name and click on Copy.
Click the Send/Receive button to move OLM emails to Gmail account.