What does this mean? Like having a good custom?
Yes, having a good custom is a short description. Like morals, manners and conventions.
Doing so is intended to behave as a "civilized person" - being recognized by other people. This refers to kind of "common understanding of Do's and Don'ts of the better situated (middle) classes".
kind of = not everybody has the same understanding what it really means. And your viewpoint of your "social environment" comes in.
Mainly regarding manners and social graces. Not to mix with something like business decisions etc.
That implies: not doing so "marks" you as a simple minded peasant/ worker class / uncivilized person etc. (which basically says that the uncivilized persons don't care about social environment - which is not to be discussed here)
in case your nose is congested / you have a cold, there are some options how to get it free:
- put your finger on one nose' side, move your head to the other and "blow out"
- snuffle the congestion back in
- use a tissue to catch your "blow out"
- take care that as less as possible people realize that you are cleaning your nose (using a tissue)
What would you say is "a good manner/ custom" here?
In my opionon: the third "gehört sich so", the fourth can be seen as exaggerated. Circumstances are important.
It is about thoughtfulness and consideration of your environment as the common understanding of the current local society. Mainly it takes care of treating other people. And how you present yourself = implications how you live your life.
es gehört sich, alten Menschen einen Sitzplatz anzubieten - to offer a seat to elderly people
es gehört sich, auf dem Fußweg nicht alle anzurempeln, sondern in gegenseitiger Rücksichtnahme - don't rush and push other people away on a sidewalk, consider to move aside
es gehört sich, beim Verlassen des Hauses ordentlich angezogen zu sein - dress well when leaving your house
"common understanding" comes to it's end easily in last example: what means "dress well"? Someone might say "clean clothes that fit", others might require no sportive, next one might require "suit". The differences start the moment you try to define "leaving the house".
That also means: if noone else is around: drop behavior or not can't be decided as "gehört sich so". That decision refers to questions of personality, living principles, human dignity and moral. etc.
Der gute Ton (the good tone) probably comes from French le bon ton. This means "good manners" or "good form". In English, a century or more ago, "le ton" was high society.
Le bon ton and der gute Ton therefore mean good manners, and probably come from the way you (should) express yourself, i.e. in which "tone".
Es gehört zum guten Ton literally means It belongs to good manners, so I guess it is clear what it means: it is good manners or good manners require.