In this work, we have performed the chemical functionalisation of metallic graphene nanoribbons (GNRs) with different functional groups. The analysis of graphene in terms of relative stability and electronic properties has been done. The HOMO–LUMO gaps are quantitatively analysed to reveal the influence of different functional groups including hydroxyl, carboxyl and hydrogen sulphide groups. Interestingly, the influence of edge functionalisation on the HOMO–LUMO gap of zig-zag graphene nanoribbons (ZGNRs) presents significant change using density functional theory (DFT). Understanding the electronic properties in terms of density of states and band structure of functionalised graphene is of great relevance today. It is found that the geometrical structures and electronic properties of the GNRs could be significantly changed with the oxygen containing group. With the carboxyl-functionalised GNRs, the interaction leads to a decrement in the HOMO–LUMO gap of graphene. This fact makes GNR a possible candidate for nanoelectronic devices.