I have never ventured into Gem until very recently, but in a quest to create an interface to crash things into each other to cause cannon effect using a mouse or touch screen. I first had to work out how to monitor areas of the screen and object locations. So as far as monitoring goes I offer this purely for its lava lamp hypnotic style and the fact someone might find it useful.
There are three versions Gem (bounce.pd), Gem with midi out (bounce-midiout.pd) and for those with no Gem a scalar graphic version (scalar-bounce.pd).
There is also an idea for Gem slider but it not scalable