There are two ways you can migrate Zoho Mail to Gmail. You can do it manually or using a professional approach. If you are migrating Zoho Mail to Gmail manually, you will have to make sure IMAP is enabled on both accounts. You will then add your Zoho account in Gmail using the Account and Import > Check Mail from other Account with Zoho's IMAP setting. Gmail will now fetch your emails to your inbox.
Bear in mind:

Manual migration will be a slow process because you will be limited to accessing your mailbox based on your (remote) syncing preference in Zoho. You will not transfer contacts or calendar data either, and even labels might be overlooked. Manual migration is inefficient when migrating very large mailboxes and even more so when constantly updating your mailbox contents.

The CloudMigration Zoho Backup Tool is a convenient way to migrate Zoho emails to multiple email clients without hassle.

Migration Method with CloudMigration
1. Install and launch the utility as an administrator and select an account mode.
2. Select Zoho and give your email ID and app password. Click Login.
3. Select the Zoho mailbox files and folders to backup from the preview panel.
4. Then click Next, and select Gmail from the options drop down.
5. Then click Next and login using your Gmail credentials.
6. Select the filters you want to use, and finally click Convert to start the Zoho Mail to Gmail Migration.
7. Now you have completed the migration process in a few simple steps.

Know More: https://www.cloudemailmigration.com/blog/migrate-zoho-mail-to-gmail/