Gel materials are materials with high elasticity that deform when stressed and then return to their original shapes once the stress is removed. Gel materials are expected to be used in artificial living tissues, soft robotics, sensors, and so on. On the other hand, humans grow and change their bodies and characteristics over time. In this research, we developed an adhesive gel system that can grow and change its characteristics like our humans after adhesion through reversible addition–fragmentation chain transfer (RAFT) polymerization, a class of living radical polymerization. The adhesive joint grown by RAFT polymerization had higher adhesive strength than the non-RAFT-polymerized adhesive joint and a function, i.e., thermal responsiveness of adhesive strength. The environment around us changes over time. These growable adhesive gel systems should enable us to use gel materials for a long time while adapting their structures and characteristics to environmental changes.