DTTG is allowed access to Photos in Settings and the “Clip to DT” icon is available via the “Share” icon.
Tapping on “Clip to DT” produces a new document within DTTG but it’s empty - no image is visible. File type is “Picture” but file size is zero bytes. Copy and pasting an image or creating a new document from Photo Library does work.
For completeness - Clip to DEVONthink does not work with images in any image-creating app that opens the sharing extensions and offers the user to use the “Clip to DEVONthink” widget. If the widget is not going to work, it should probably be disabled in apps like TouchDraw, iDraw, Paper by 53, etc.