The garnet-type Li7La3Zr2O12 (LLZO) is deemed as the promising solid electrolyte for solid lithium batteries due to its high stability and ionic conductivity. However, the rigid and brittle natures of LLZO lead to severe interfacial issues from both the cathode and the Li-metal anode sides, as well as limited applications in flexible electronics. In this work, the in-situ solidified gel polymer electrolytes (GPEs) are constructed as not only the interlayer to buffer electrode/garnet interfaces, but also the adhesive to join garnet blocks together, realizing the scale expansion and good flexibility of solid garnet batteries. For interface engineering, such GPE is conformally solidified at electrode/garnet interfaces, along with the cathode particles fully surrounded by solidified electrolytes. On anode sides, the Li/garnet interfacial resistance notably drops to 88 Ω cm2, accompanied by the symmetric cell stably cycling over 400 h with robust solid electrolyte interphases. On cathode sides, the cross-linked ion-conducting network is formed inside the cathode, and the intimate contact is built at cathode/garnet interface, arousing remarkable cycling and rate performance for LiFe0.2Mn0.8PO4/Li and LiFePO4/Li cells at 30 °C. Furthermore, the soft GPE can be used as adhesive to connect garnet blocks together without gaps, enabling the construction of flexible and large-scale solid batteries, in which the GPE functions as deformable medium to induce the bending and coiling of solid batteries. This work demonstrates a promising strategy to address both the rigidity and brittleness issues of garnets by utilizing the in-situ solidified multifunctional GPEs.