Welcome to bowhunting and welcome to the forums. To answer your question, a gilly suit probably wouldn't work because you still have to draw your bow and that will cause movement. If there are no suitable trees, I would suggest a ground blind or an elevated enclosed blind. In either case, you will want to get it out there very soon as the deer will need time to become used to it's presence. Are there any woods that borders this tall grass that you have permission to hunt? The deer have to be coming from somewhere. Try looking at the property in an aerial view, such as Google maps. That might give you a better idea of what to do. Hope this helps.

Blessings.....Pastorjim