It could be because you're female. Tons of people think all gsds are "police dogs that bite people and are extremely hard to control" - at least in my area.
If you come across as really friendly or gentle people will assume your dog will be bossing you around when they're larger. I'm a gentle person, pretty quiet, 5'4" woman..I sometimes get looks like I am crazy when I tell them I have a gsd because they don't expect me to be a firm handler, they expect me to be overly sensitive and forgiving for my dog's misbehavior.
I wouldnt think much of it, for all you know, that guy hard a hard time training his lab and is jealous that your gsd is so well behaved at this age and it made him feel worse to find out a woman is a better handler. You never know!