The trick is to use the “last_buffer” property on your video sink. That contains the last frame that was displayed to your screen.

You can use RMagick to convert that into something you can use like this:

#!/usr/bin/ruby

require 'gst'
require 'RMagick'

@pipe = Gst::Pipeline.new
@source = Gst::ElementFactory.make("v4l2src")
@sink    = Gst::ElementFactory.make("xvimagesink")
@pipe.add(@source,@sink)
@source >> @sink

@pipe.play

[ ... Later ... ]
# This is the actual capture.
capture = Image.from_blob(@sink.last_buffer.data).first