Question

example of a gas reducing agent

Answer 1

Some compounds can act as either oxidizing agents or reducing agents. One example is hydrogengas, which acts as an oxidizing agent when it combines with metals and as a reducing agent when it reacts with nonmetals. Another example is hydrogen peroxide, in which the oxygen atom is in the -1 oxidation state.

Answer 2

Lithium aluminium hydride (LiAlH4), a very strong reducing agent.
Nascent (atomic) hydrogen.
Hydrogen without or with a suitable catalyst e.g. a Lindlar catalyst.
Sodium amalgam (Na(Hg))
Sodium-lead alloy (Na + Pb)
Amalgam (chemistry)#Zinc amalgam (Zn(Hg)) (reagent for Clemmensen reduction)
Diborane.