BAHCO SH02060 4.5x4.5 USP Single Acting Spring Hinge, Full Mortise, Standard Weight, Steel Base, Primed
Is indeed available in stainless steel and is the
BAHCO SH02060-32D 4.5x4.5in Single Acting Spring Hinge-Full Mortise- Standard Weight-Stainless Steel Base as seen here:
Click Here
Is it is reasonably priced and will handle the weight and thickness of the gate.
We can indeed sell you the same concept as a stainless steel base material but expect the price to go up by a factor of 4. This hinge would be the JDA-180-38S, DOUBLE ACTION SPRING HINGE 5-1/4" W SS aAs manufactured by Sugatsune and seen here: