Emails backed up on external hard-drive, how do I transfer those emails into a new gmail account?

Assuming they were using Outlook and messages are stored as *.pst files...

1. Setup IMAP access in the Gmail account and connect it to Outlook: Instructions
2. In Outlook, open the old Outlook Data (pst) File: How to
3. In Outlook favorites bar, both the old and new accounts inboxes, sent items will be visible. Copy / paste or drag and drop old messages in to new accounts location. Instead of doing it manually, it is also possible for Outlook to automatically import / export, but I prefer doing the first way..
4. Disable IMAP access in the G-Suite / Gmail account if it is not needed.