As it is a coastal town, fashion in Gisborne is highly motivated by the beach. Cool clothing is a must - especially as it will be hot! Comfy sandals are what everyone will be wearing, and light clothing - T-shirt/shorts/skirts.
Definitely bring sunglasses, you WILL need them!
Toiletries and Medical Supplies: Some may think, oh yeah, thats a given, but it really is important. The burn time in New Zealand, especially in the height of summer can be as little as a couple of minutes, and without that ozone layer up there to stop those rays, you'll be doing yourself damage if you don't cover up