If you mean Gnotuk Avenue, you'll be OK. Go into the carpark and turn left, then go to the end. You'll be able to rig up on the needle grass ... difficult but do-able. When sailing, stay to the left of the yellow pole..... the area to the right is a swimming area only.
Gnotuk is very popular with kitesurfers, so you'll need to keep away from them, if any turn up.
Gnotuk is no longer popular with windsurfers, mainly due to the difficulty in rigging, so you may not have anyone who you can ask for help, if needed. You might also consider Williams Ave in Bon Beach, which has a resident population of windsurfers, although the forecast winds will probably not excite the normal crew, so they'll stay away. If you go there, go down Williams Ave, turn right at the car park, then go along to the end of the car park, There's a better rigging area there than at Gnotuk.
Note that both Gnotuk and Bon Beach car parks are fee-payable and in warm weather, the parking officers often come around 3 times in a day.