I got a very helpful reply on the Pd mailing list:
Basically there were two completely separate problems in my two installations, despite their identical symptoms. On the Raspberry Pi, I was just missing the gem-plugin-* packages from the Raspbian repos. Loading and playing videos works fine there now.
As for my Laptop, I was advised not to rely on deken for installing Gem. At the moment, I'm trying to compile it from source, as there is no package for it in the Arch repos.
On the Laptop, I have tried a few different formats already, including the "alea.mpg" included with the example patches. And the first error message shows up before any attempt to open a file, so I figured it shouldn't be about the format. Or can it? I'll try with a few more tomorrow.
Update: Tried all the bundled test files (homer.avi, anim-1.mov, alea.mpg) and a few others, none worked.
I'm a complete beginner with Gem and very inexperienced with Pd, so please bear with me.
I'm having trouble loading video files using [pix_movie] or [pix_film]. Creating or loading these objects leads to the error message:
[pix_movie]: no movie decoding backends found!
And subsequently when I try to open a video file with an [open( message, this fails with:
[pix_movie]: unable to open file: ../data/alea.mpg
Apart from that, Gem appears to be working fine.
This happens to me in two different installations. In one case on a Raspberry Pi 3 B running DietPi as an operating system, Pd 0.47.1 and Gem 0.93.3 installed as Raspbian packages. In the other case on my laptop running Arch Linux, with Pd 0.49.0 installed from the Arch repos and Gem 0.87 installed via deken. Given these very different contexts, I strongly suspect that there is no bug, but that I'm missing some configuration that I have to make. However I cannot figure out what.
Any pointers welcome!