Star-shaped polymeric ionic liquids (PIL-3 and PIL-4) based on poly(2-(dimethyl-ethyl-amino)ethyl methacrylate) bis(trifluoromethylsulfonyl)imide (P(EtDMAEMA-TFSI)) with three-arm and four-arm architectures were prepared through atom transfer radical polymerization (ATRP) and ion exchange. Subsequently, gel polymer electrolytes (GPEs) comprising PIL host, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) ionic liquid (IL) plasticizer with the same anion unit and LiTFSI salt were obtained. Compared with the linear PIL possessing the same elements, the increase of branches for PIL decreases the crystallity and glass transition temperature (Tg) of the corresponding GPEs. Accordingly, the ionic conductivity, electrochemical window and ion transport number of three-arm and four-arm PILs based GPEs (PIL-GPE-3 and PIL-GPE-4) are obviously superior to the linear contrastive sample. Among lithium metal batteries assembled by linear, three-arm and four-arm PILs based GPEs, Li/PIL-GPE-4/LiFePO4 presents the best performance. The specific discharge capacity attains 151 mAh g−1 at 0.1 C, the capacity retention ratio achieves 93.6% after 100 cycles at 60 °C, and the coulombic efficiency is around 99%. The outstanding compatibility between PIL-4 and IL and the resulted interfacial stability between PIL-GPE-4 and electrodes both contribute to the excellent performance.