Google drive is used to store data, pictures and other files and also to take backups of applications. If you want to take backup of whatsapp on google drive, you should have registered email id and password. For this first login to your google drive and then click on 3 horizontal bars on left corner and then click on backups . Here, all backup of your WhatsApp account is stored. You can manage this backup frequency from setting option available just below the backups option.
It doesn't matter that you don't have the same WhatsApp number.
Further, as you have mentioned that you have access to your google drive and you already have taken backup your WhatsApp data to your google drive, you can download backup of whatsapp chat, media and other data directly from Google drive.
1. For this you will have to uninstall the WhatsApp app from your mobile and
2. Then reinstall whatsapp.
3. Now verify mobile number.
4. After verification app will ask for restore chat messages and other files from Google drive.
5. After restoration is completed, you will be able to see your all chats and media files in whatsapp.