When you download a media in WhatsApp, it will be automatically saved to your phone gallery. However, you can stop the media from being displayed in your phone's gallery. To do so, you will need to create a .nomedia file in the WhatsApp images folder. Note: this will hide all of your WhatsApp photos from the gallery.
First, you will need to have a file explorer. You can download a file explorer from the Google Play Store.
After downloading a file explorer, you will need to browse to WhatsApp/Media/WhatsApp Images/ and create a file named .nomedia (the period is necessary).
If you wish to view your photos in the gallery later, simply delete the .nomedia file.