괜찮아: basic form and used for close relationship 괜찮아요: '~요' is kind of speaking stlye representing informal politeness. 괞찮네요: '~네' is used to express something that you just realize.

괜찮아고 is wrong. 괜찮아 is able to use only for younger people or very close people like best friend or close friend. 괜찮아요 , 괜찮네요 are able to use for elder people or not close people for polite. If you say just 괜찮아 to elder Korean or some Korean who are not close to you then they think about you're very rude. And sometimes 괜찮네요 sounds a little bit sarcastic. For example, your friend ask your opinion about her dress. But you don't think it's perfect but not too bad. Then you can just say 괜찮네.. or 괜찮네요.