Learn English – the meaning of the ` cya` and `gng`

SHORT VERSION:
Everybody, and nobody.

LONG VERSION:
Some countries have formal institutions which lay down rules which state what language is considered proper. English does not. All speakers and writers get a vote, and may exercise that vote as many times as they wish, both by speaking or writing themselves and by hearing and reading and approving what others speak or write. It is the most completely democratic institution in the world.

This is not to say there are no rules. There are, many thousands of them. Language at bottom is nothing but rules.

But the rules govern within larger and smaller communities. The rule which says that the present indicative form of the verb be which is used with the pronoun he shall be is is very widespread, but there are substantial communities where be is employed in some or all circumstances. By contrast, the community which maintains the rule that shall shall be employed in the first person where will is called for in the second and third persons is small and growing continually smaller. And the community which holds that my brother shall be referred to and addressed as Quid is quite small: probably only nine people.

So when you ask this question—which is an important one, and passionately debated—you must also specify ‘correct to whom’? To whom are you speaking or writing?

And when you have specified to whom you will have answered your question. They determine what is correct English.

Both "dude" and "man" are INFORMAL.

Whether or not they are disrespectful depends on whether you are expected to have a formal or informal relationship with the person you are addressing. If you have a familiar relationship already, calling them either term reinforces that familiarity. If I say to my friend "Check this out, man!" the subtext of the message is that our relationship is such that we can use familiar terms, and it strengthens the relationship.

On the other hand if I find a traffic policeman writing me a parking ticket, and I say "You can't do that, man!" then it is disrespectful. By using an informal term in a situation that calls for formality, I am refusing to acknowledge the authority of the other person and thus disrespecting their position.

Also, a lot depends on the TONE with which you say them. As with any sentence, an aggressive tone makes the same word seem more offensive than a friendly tone.

See, for example: https://www.youtube.com/watch?v=77v_Q0mhbZU (0:00 to 0:45)