There is always plasma glucose wheather from gluconogenesis or absorbed dietry glucose. The GIP increases insulin sensitivity of fat cells, increasing liposysis if needed, increased hepatic fat mobilisation, increases hypothalamus central appetite satiety signals and buffers the glucagon induced glucose production. I still cannot see any evidence that even moderate carbohydrates are "needed". If dietry intake of carbohydrates is low the GIP and glucagon increases the liberation of glycerol from fat cells into triglycerides and converted into glycerol which is used for gluconogenesis in liver etc if needed or glycolysis directly in the cells for energy. It allow better partitioning of dietry glucose but saying you "need " dietry carbohydrate for GIP to "work" doesn't make sense. Dietry glucose efficiency is only a part of its benefit.