I remember there was a bug with GridLines only showing horizontal lines and not vertical when using Ticks. Any one remember if this is known? I was going to post it, but thought to check here.
Plot[Sin[x],{x,0,Pi},
Ticks->{{0,Pi/4,Pi/2,3/4 Pi,Pi},Automatic},
GridLines->Automatic,GridLinesStyle->LightGray]