I was considering a used Mac Mini, but a RPI seems possible as well. I am aware it would be pushing the limits of the RPI, but would it be feasible at all? Other than the CPU strain, I have read elsewhere that the OpenGL status of the RPI is hazy for GEM support

I remember having GUI issues with pd 0.47 and with the beta driver enabled.

you can either use [declare -stdlib Gem] in your patch or load Gem on startup from File/Preferences/Startup.

if you are having trouble installing pd itself these are the steps i followed to install:
most of this is from the README.txt and INSTALL.txt files in the Pd folder. you can check those files for more information.)

Start by opening a commandline shell:
and entering the following:

sudo apt-get install build-essentials automake autoconf libtool gettext

sudo apt-get install libasound2-dev

then navigate to the Pd source directory:

    cd path/to/pd

./autogen.sh
./configure 

make
sudo make install

Additionally I did have to install tcl/tk using:

sudo apt-get install tk

you can then open pd from the command line
by entering
pd

Better use raspbian Stretch I think, because in older Raspbian version, pd and OpenGl rendering looks buggy in the patch's view (Maybe TCL graphic rendering problem: no pathchcoords )

p.s. still having to bang the gemhead to actually render frames when using pix_video. is there another message to automatically play at framerate ? toggling the gemhead does nothing..

I just installed pd vanilla on my Raspberry Pi 3 and managed to get Gem running at normal speed, no need to make any changes to my system.