Frankly, I find the bold statment wrong.
Who are we to put a limit on Allah’s mercy and forgiveness?
Is there a limit according to our Aqeedah?
When Allah:subhanahu: Himself addresses this issue in Quran [something I always refer to Divine Orations] saying:
39:53] Say: "O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah. for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful.
Then, how does such question can even be arose?
Any sin (even shirk) will be forgiven IF and only IF we ask for forgiveness while living in this world. After death, any sin can still be forgiven except shirk.
This Taubat An Nasooha (Sincere repentance) is also addressed in Surah At Tehreem:
66:8] O ye who believe! Turn to Allah with sincere repentance: In the hope that your Lord will remove from you your ills and admit you to Gardens beneath which Rivers flow,- the Day that Allah will not permit to be humiliated the Prophet and those who believe with him. Their Light will run forward before them and by their right hands, while they say, “Our Lord! Perfect our Light for us, and grant us Forgiveness: for Thou hast power over all things.”
One point I want to make here though…
You mentioned “gossip,” “back biting,” etc. When we do that, we are violating fellow humans’ rights… Allah is JUST. He will let this to be decided by the person whose rights were violated.
So it is very very imperative that we understand this. Often, we do not give importance to other people in this world. We never know what will take us to the Naar-e-Jahannam…
Worst thing that can be done to our ownself is to back bite others.
Result is simple in Allah’s eyes: our good deeds will be given to them PLUS we may end up getting their sins as well.
This is about back biting - something which might be true about the person…
We can always think about “bohtaan” … what will be the punishment for that?
So, it’s Allah’s mercy that we sometimes reflect upon things…
Reflecting upon may not be enough… Asking for forgiveness is the only way…
May Allah 
guide all of us. Aameen.