Why did a gate post rot after just 6 years?
The most common failure with posts in concrete is "collar rot", where the post rots right at the point where it exits the concrete at the ground line. This can be easily avoided if the concrete is slightly above ground grade and domed or tapered away from the post so water doesn't lay against the wood. Drainage is the key, keep standing water away from the posts and they will last a lot longer.