Since Gem uses accelerated OpenGL, you would need to hack Gem in order to write video frames on the disk,one by one, using offscreen rendering: http://www.mesa3d.org/brianp/sig97/offscrn.htm The easiest solution is to use a vga to composite converter (or a video card that has a composite output) and grab the video from the converter output. I used this technique a few times.
is there anything that can write videos to disk directly out of pd/gem?
well, maybe use [pix_write] ? it works much the same as [pix_snap] but writes data the current frame to disk, either as jpeg or as tiff. unfortunately it is not supported under macOS right now (i think)
mfg.as.dr IOhannes