Gattermann-Koch reaction - Role of cuprous chloride
According to this site:


  The formylation of aromatic compound can be accomplished by treatment with $\ce{CO}$ and $\ce{HCl}$ in the presence of Lewis acid. The reaction is performed either under pressure or using copper(I) chloride or anhydrous aluminum chloride whose role may be to aid the reaction between $\ce{CO}$ and $\ce{HCl}$ via the complex which it forms with $\ce{CO}$.
  
  


As you can see, it does not involve in the mechanism (provide $\ce{Cl-}$ via dissociation of $\ce{CuCl}$) but act as a promoter to speed up the reaction.