How to access my gmail account in my iCloud mail account

You don’t access another email service account from your iCloud account as one account has nothing to do with the other.

You can set up that gmail account in the iOS mail app (Add an email account to your iPhone, iPad, or iPod touch - Apple Support) or download the free gmail app from the App Store and use that for the gmail account access.

You cannot do that. Gmail is an email service from Google, and iCloud is an email service from Apple. You cannot access an email account with Google through your Apple email account. That’s like asking to access your Chase Bank account from your Bank of America bank account - separate accounts with completely independent companies do not share access to each other.

separate email services, separate accounts. You can access both through the iOS email app, but you cannot access one account through the other.

You can login to your gmail account from a web browser and go to settings to have your gmail emails forwarded to your iCloud email address so they appear in your iCloud account inbox. But when you reply to those the replies will be coming from your iCloud email address.