The influence of adsorbed impurity of potassium atoms on the electronic spectrum of graphene is investigated. Electron states of the system are described within the framework of the self-consistent multiband tight-binding model. As shown, for the ordered arrangement of potassium atoms, which reduces energy of the system at issue, a gap can arise in the energy spectrum of graphene. As determined, at the potassium concentration such that the unit cell includes two carbon atoms and one potassium atom, the latter being placed on the graphene surface above a carbon atom at a height of 0.286 nm, the energy gap is equal to $\cong$ 0.25 eV. Such a circumstance is realized, if graphene is placed on a potassium substrate.