Anything you download from Whatsapp is going to take up space on your phone. Your photos are going into Whatsapp's 'Images' folder (which is inside its 'Media' one). When Google Photos or Android Gallery search for images they include that location as one to look at. (i.e. the photos AREN'T being copied into a separate folder used by one or other of those apps; they're ONLY in the Whatsapp 'Images' folder).
You can stop Google Photos and Android Gallery looking in the Whatsapp folder for images by following the instructions under 'Hide WhatsApp images from Google Photos or Android Gallery' here:
https://www.techjunkie.com/stop-whatsapp-saving-videos-photos/
However that won't reduce the amount of space that they take up on your phone because they'll still be in the Whatsapp 'Images' folder (where they have to be if you want to continue having access to them through Whatsapp).