There doesn't seem to be a way to remove images from a gallery in the new image manager without completely deleting the image. The drop down options in a gallery when selecting an image include a delete function but this appears to completely remove the image as it would do in the full file view and there is nothing I can find that just allows me to take the image out of the gallery.
I opened up my gallery from within the image manager and then attempted to find a way to remove one of the images in it but couldn't see anything in the interface that seemed to serve this purpose, except delete which removes the image completely. This was possible to do safely with the old interface.
This is an oversight, where the 3-dots dropdown does not include the option to remove from gallery. Right-clicking the image (on desktop) or long-pressing the image (on mobile) should show the option. I'll work on adding that option to the normal 3-dot menu there.
I have corrected this, and it will be shipped with the next update!