Robust freestanding polymer gel electrolytes (PGEs) are fabricated via a facile approach and used for flexible zinc-air (Zn-air) and aluminium-air (Al-air) batteries. The fabrication involves an ultraviolet light-initiated free radical polymerization of acrylamide using N,N′-methylenebis (acrylamide) as crosslinker to form a partially crosslinked polyacrylamide, and the soaking of the resultant polymer in a potassium hydroxide aqueous solution or a saline. With their merits of good flexibility, high ionic conductivity (σ = 0.33 S cm−1), easy fabrication and scalability, these freestanding PGEs qualify as superb candidates to enable high performance flexible metal-air batteries. The Zn-air batteries show no noticeable performance degradation at a bending angle down to 60°, delivering a power density of 39 mW cm−2 and a stable voltage profile over 50 h of continuous cycling (1 h per cycle) with a discharge and charge potential of 1.2 and 2.0 V, respectively. The flexible Al-air battery assembled using a neutral pH PGE and a household aluminium foil anode gives a high areal specific capacity of up to 20 mAh cm−2.