What is the action of hydrogen gas on cyanogen?

Given that a nitrile (#"C"-="N"#) like cyanogen is similar to a comparable imine (which has #"C"="N"# bonds instead), #"H"_2# should behave similarly to how it behaves with imines.

It reduces imines into amines, so it should reduce nitriles (#"C"-="N"#) into imines (#"C"="NH"#), and then amines (#"C"-"NH"_2#), as it indeed shows here.

This parallels the reduction of alkynes (#"C"-="C"#) into alkenes (#"C"="C"#), and then alkanes (#"C"-"C"#).

These types of hydrogenation reactions require a catalyst to cleave the #"H"_2# bond such that it can add cis to the substrate compound (in this case, the nitrile). Here is an example:

It likely depends on the catalyst whether or not a single #pi# bond will be removed, or all of them.

I will assume a palladium catalyst over carbon (as you would traditionally use on alkenes and alkynes for full reduction), and that the catalyst promotes reduction of one #pi# bond at a time without reducing all the way in one go.

#"N"-="C"-"C"-="N" stackrel("H"_2" "" ")(->) "N"-="C"-"CH"="NH"#
#color(white)(ssaaaaaaaaaaaa)""^("Pd/C")#

#"N"-="C"-"C"-="N" stackrel(4"H"_2" "" ")(->) "H"_2"N"-("CH"_2)_2-"NH"_2#
#color(white)(ssaaaaaaaaaaaa)""^("Pd/C")#

Either way, it accomplishes the same result at each step, just multiple times where applicable.

For the reduction of nitriles, elevated temperatures and pressures promote the reduction.