The $\langle1\bar 100\rangle$ edge dislocation in GaN has been investigated by means of atomistic computer simulation. It's core structure exhibits three configurations that are related to the location of the origin of displacements imposed to the atoms when generating the dislocation in the initial configuration. They are described by: (a) 8- and 4-coordinated channels (8/4 core); (b) 8-, 5-, and 5-coordinated channels (8/5/5 core); and (c) 9- and 5-coordinated channels (9/5 core). An additional configuration where the $\langle1\bar 100\rangle$ dislocation decomposes in two 8-coordinated channels a= 1/3$\langle1\bar 210\rangle$ dislocations is also generated and it exhibits the lowest energy. This is in agreement with high resolution transmission electron microscopy observations and image simulations carried out on an asymmetric Σ7 [0001] tilt grain boundary in which only cores of elemental a type dislocations have been determined.