Earth is rotating on an axis.its an imaginary one. Is there any relation b/w zero longitude and the earths axis? Are they same?
Can you explain the working of GPS and how the latitude and longitude are using in it?

no there is no relation between 0 longitude and axis of earth .

and no they are not same .Axis is a line which goes through centre of a sphere and we can rotate the sphere around it .

as for imaginary line you can take axis as a line going from north pole to south pole through centre of the earth .

whereas longitude can be taken as lines going from north to south pole on surface of earth .

if you take where you are standing right now, as a point on earth ,there can be a line drawn from north to south pole passing through you ,which is longitude .

and another line perpendicular to it ,which is latitude . so that how your position is decided .