How can I restore my old chats on WhatsApp from a different phone number but the same Gmail account?
WhatsApp keeps its chats on your internal storage in the folder /WhatsApp/Databases. There might be a large number of files, the latest being the one with no date in it (msgstore.db.crypt12, probably).
I'm not sure if the phone number being different matters if you have not enabled WhatsApp's 2FA, and if you have enabled cloud backups to Google Drive as long as you use the same Google Account you should be okay.
If you have the old phone, you could just copy the entire WhatsApp folder from old phone internal storage to new phone internal storage.
When you install WhatsApp on new phone, during the process, it will ask for restore from backup. Selecting YES will automatically find the backup and restore.
If you have it on your Google drive, you could just download it your new phone internal storage before installing WhatsApp and follow the same procedure.