do I need anything to reach the gnu I can't seem to jump to the top
If you've defeated Mushna, you will receive the umbrellashroom which will allow you to travel on updraft currents, making it possible to explore more areas of the map.
Once you find the area where the gnu girl is, you'll need to find the mushroom dog, give the dog any mushroom to make the dog follow you and then bring the dog to the gnu girl area to activate the boss battle. Hope that helps