The function of the gap junction is to

Gap junctions facilitate communication between neighbouring cells via rapid transfer of ions and molecules. Gap junctions are formed by two adjacent cells that are joined together by channels called connexons. These channels allow small molecules and ions to pass freely between the cells, allowing them to communicate
and exchange substances. 
(1) Gap junctions do not separate but form a communication between two cells.
(2) Tight junctions stop leakage of substances across an epithelium. 
(3) Adhering junction performs the function of cementing to keep neighboring cells together.