The practical way to migrate Gmail to Zoho Mail is to transfer the mailbox directly through IMAP rather than downloading and uploading the emails manually.
I had this situation with a Gmail account containing years of emails, custom folders, and attachments. Manually moving that much data would have taken too much time, so I used GainTools Gmail to Zoho Mail Migration Tool to transfer the mailbox directly. I could select the folders I needed and apply filters before starting the migration.
I first moved a smaller date range and checked the emails, folders, and attachments in Zoho. Once everything looked correct, I continued with the older data. For a large Gmail mailbox, this staged approach is easier to verify than moving everything at once.