Thumbnails not working

I’m looking to set up a page with a gallery of PDF newsletters. I have installed ghostscript in the container (ubuntu 14.04) default location /usr/bin/gs. However, DG can’t find it unless I put the path into the advanced page.

When I open the page, itjust puts the default PDF thumbnail on the files. I have used the code posted elsewhere to test that gs works from the command line and I see that temp png files are being created in the uploads folder when I open the page, but in the log after the ‘Attempting to generate thumbnail for attachment …..line I get (DG_Thumber::getThumbnailTemplate) Failed to get image editor: No editor could be selected.
I imagine the fact that it couldn’t be found automatically and the subsequent failure could be related. Can you suggest something else to try? I’m aware that googledrive is not working currently.

Thanks

https://wordpress.org/plugins/document-gallery/