It looks like Ashton never implemented the Window#to_image it was on the TODO list on the wiki
https://github.com/Spooner/ashton/wikiI cloned the project and took a look hoping it was there, but sadly no.
Maybe the opengl gem could be used to do this piece as bestguigui suggested, although I have not worked with that gem in the past and am not quite clear on how I would convert that to a Gosu image so as to be able to save it.