Z. Gefgs. = ?

[WaPrüf2]

This stamp found on both sides of the butt of a so-called Gewehr 98M reworked sometime after 1933.

[vkun]

May be it means "Zentralgefängnis" (main prison).
Googling "zentral gefängnis deutsches reich" I found
http://www.shoa.de/holocaust/ghettos/45 ... -lodz.html
There was a Zentralgefängis in Lodz in 1941(search for Zentralgefängnis on the site mentioned above). Maybe the rifle belonged to the guards.

[WaPrüf2]

Good possibility. One wonders if this is a generic or specific term.