Thomas,
You need to reverse things a bit. Specify the Gridlines and use Ticks->Automatic:
For example:
Plot[Sin[x], {x, 0, 2* Pi},
GridLines -> {Range[0, 2 Pi, Pi/2], Automatic},
Ticks -> Automatic]
or Specify both Gridlines and Ticks if you want to control the ticks more precisely, such as:
Plot[Sin[x], {x, 0, 2* Pi},
GridLines -> {Range[0, 2 Pi, Pi/2], Automatic},
Ticks -> {Range[0, 2 Pi, Pi/2], Automatic}]
Regards,
Neil
