Ya, you can spot the G-Man in tons of places. But why are you talking about a cage? Are you thinking about Lamars basket, or is there something else?
The only place I remember seeing the G-Man in Kleiners lab was on the TV monitor (once you select it to change the channel).
PS. Aperture science is creepy, Id feel so much safer with the friendly people of Black Mesa
(And its called 'Garry's Mod' - not garys. Sorry but I felt like correcting that)