Suppose I'm discussing about a person in a gathering but I am not revealing that person's identity i.e. there is no way for the people in the gathering to know who that person is.
Also, the people in the gathering have never met that person being discussed about.
Will this still be considered as backbiting?
If there's any problem in my question please inform me. Thanks!
If it is like you've explained it has good chances not to be backbiting.
As backbiting is defined in the sunnah in:
Do you know what is backbiting? They (the Companions) said: Allah and His Messenger know best. Thereupon he (the Holy Prophet) said: Backbiting implies your talking about your brother in a manner which he does not like. It was said to him: What is your opinion about this that if I actually find (that failing) in my brother which I made a mention of? He said: If (that failing) is actually found (in him) what you assert, you in fact backbited him, and if that is not in him it is a slander. (See for example in sahih Muslim)
Malik related to me from al-Walid ibn Abdullah ibn Sayyad that al-Muttalib ibn Abdullah ibn Hantab al-Makhzumi informed him that a man asked the Messenger of Allah, may Allah bless him and grant him peace, "What is backbiting?" The Messenger of Allah, may Allah bless him and grant him peace, said, "It is to mention about a man what he does not want to hear." He said, "Messenger of Allah! Even if it is true?" The Messenger of Allah, may Allah bless him and grant him peace, said, "If you utter something false, then it is slander." (al-Muwatta')
But there are cases where backbiting is allowed (most are related to shari'a circumstances and covered in Should we speak about someone (when he is bad, and I'm sure about that) to aware others?).
Back to your case we also find a hadith where the prophet () says:
What has happened to these people that they say so and so, whereas I observe prayer and sleep too; I observe fast and suspend observing them; I marry women also? And he who turns away from my Sunnah, he has no relation with Me (See for example sahih Muslim)
As you described he () didn't name any of these people and others didn't know who they were. This hadith is used as an evidence to say that if you didn't mention the person you are talking about nor the audience is able to recognize this person this is not the backbiting which is haram by the quran (49:12) and sunnah (see the ahadith above).
The prophet (Allah bless him and give him peace) said,
“Backbiting is to mention your brother with that which he dislikes.” [Muslim, Tirmidhi, Abu Dawud]
Imam al-Barkawi said in al-Tariqa al-Muhammadiyya:
Backbiting is to mention the negative points of a specific brother who is known to those being addressed, or to indicate these points or make them known by the hand or any other limbs
Allama Abu Sa`id al-Khadimi explained:
“(A specific brother who is known to those one being addressed) for the who is not specified or known would not be backbiting.” [al-Bariqa al-Mahmudiyya fi Sharh al-Tariqa al-Muhammadiyya, 3: 183)
However brother it is safer not to talk about any one unless with good as the prohpet said:
“Whoever believes in Allah and the Last Day should say that which is good or remain silent.”
Bukhary 73:158