Gheebat is basically undermining someone’s reputation etc. in his absence where he is not able to defend himself. Thats why backbiting is equated to eating flesh of a dead brother.
Here are the quranic verses
I think it is better to avoid such discussions. But if the matter is serious and someone asks your opinion in a matter like initiating a relationship then I think it is good to provide ‘awareness’ to the concerned party but without any exaggeration. I think in that case, it would not be ‘gheebat’ because your motive will be to save someone from committing a blunder. but I would like to hear from others if they have differing opinion in this case.