Mechanism of the Gatterman Reaction
The Gattermann reaction you mentioned in the question uses Copper powder and Hydrohalous acid to substitute diazonium group with a halide in an aromatic diazonium salt.
It uses $ArSN_1$  pathway, similar to Sandmeyer Reaction which uses Copper Halide and corresponding Hydrohalous acid instead of copper powder.
As diazonium group is a very good leaving group, it departs leading to a phenyl carbocation which is very unstable, this is in fact the slow step of the reaction. 
The Nucleophile (Halide in this case) attacks the phenyl carbocation leading to the formation of Halobenzene(fast step).
Nitrogen gas and Hydrogen chloride are also formed.