I've been thinking how to best start doing simple, free, quick and dirty 360° video(-stitching/rendering) for VR-HMDs
(perhaps even with some simple video synthesis) and I thought about PD/GEM.

Are there any pre-existing patches for my specific application out there?
(various footage sources -> image processing -> stitching -> rendering)

Just gotten into PD on the audio side this year,
I'm not that familiar with PD itself, let alone GEM.

So please excuse such a broad question.

Greetings,
random_pulse

check out extended-view-toolkit for the stitching part.

http://extendedview.mur.at/index.html