ibrouwer no, you can't use JoomGallery for Joomla 3 in your new Joomla 4 website. Just copying files will cause many issue and it simply won't work. You need to wait for JoomGallery 4 to be released.
Thanks for the answer. I think I have not correctly explained myself: I copied the picture galleries (Originals, Details and Thumbnails), not the code files. How can I show the pictures from those folders as an interim solution to the final Joomla4 version ?