In Gattermann-Koch reaction, benzene or its derivative is treated with CO and HCl in the presence of anhydrous aluminium chloride to produce benzaldehyde. In this reaction, CO and HCl combine to form HCOCl, i.e. formyl chloride which gets substituted on benzene by the replacement of H-atom. Hence, we can say acyl group is transferred.
While in Friedel-Crafts acylation, acyl group is also transferred to carbon of benzene and acyl benzene is obtained as a result.This can be done by reaction of benzene with acyl chloride
Further, formyl chloride is unstable in nature. So, it can be prepared in situ by treating CO and HCl in the presence of anhyd. by only Gattermann-Koch reaction not by Friedel-Crafts reaction. Since, in both the reactions, acylation takes place, hence, Gattermann-Koch reaction can be considered similar to Friedel-Crafts acylation.