Oh, that's bad. It seems I cannot even downgrade back to the older version. Thanks anyway.
I've upgraded my Xubuntu to the latest LTS version which also updated the graphicsmagick. I have no idea what was the previous GM version as I haven't installed it manually, sorry. Sorry about asking, but is there any quick workaround for this (apart from specifying the dimensions)? I use GM to dynamically generate images on a website and now it stopped working.
Using "0,0" for image size throws an error in v1.3.38, worked previously