Downloading the photos, keeping the names the same and putting them back in the
folder will work for most of them. Some photos might have been resized or other meta data changed during the upload to google photos and so it might not work for all.
If you are talking about a large number of photos, then you can download ALL your google photos from here (could take a while):
to your PC, and then select all the ones with a WhatsApp-style name, e.g:
If you have set up google drive backup (Settings -> Chats -> Chat backup), then you might stand a chance by uninstalling WhatsApp, deleting the local backup from the phone, and allowing WhatsApp to retrive the backup from Google Drive. Before going to all that effort however, it is probably worth checking the size of the backup listed under (Settings -> Chats -> Chat backup). If it is large then it might contain all your photos. If it is small then it is unlikely to be worth the effort.