All I know is that whenever I was greeted with a “gut shabbos” from a guy who was either an older teen or 20’s, it wasn’t coming from just… ahavas yisroel. Let’s put it that way.
But if you’re not referring to people close in age like that, It should be fine.
But either way, don’t ignore! You don’t have to say Gut Shabbos to everyone, but if someone greeted you, you should greet them back. THAT’s where politeness comes in.