I have met some really lovely people at dog parks and some of the most rude nastiest people I have ever encountered. There are people all over the world that seem to dislike GSDs so much and get off on making the owner feel inadequate and like a leper. Don't let his nasty attitude get to you, in my experience the most nastiest rude people are the ones that have no control over their dog so will lash out at easy targets like GSD owners.
The thing with GSDs is that they are rough noisy players and if some people are not used to their style of play, they often think they are being aggressive. I have always had 2 GSDs so am experienced with how rough and noisy they are. The other thing to consider is that most GSDs do fine in dog park situations up to about the 1yo mark, some a little before, and at this age they develop definite likes and dislikes and their tolerance becomes shorter and more defined, their confidence has increased and you will find they will have no problem telling an 'in your face' Lab to go away!
I have used dog parks as a form of social exercise when they were all young pups (depending on the dogs already there) and at about 8 to 12mo I don't take them in as they don't need social exercise, but do need physical exercise so do other things with them such as ball play in a training pen, walking, hiking, swimming etc.
Do yourself a favour and ditch the dog park, find other more exciting and fulfilling things to do together - your bond together will only grow and don't worry about this ejit, some people get off on upsetting others.