قِيلَ ادْخُلُوا أَبْوَابَ جَهَنَّمَ خَالِدِينَ فِيهَا
...
It will be said (to them): "Enter you the gates of Hell, to abide therein..."
means everyone who sees them and the situation they are in, will testify that they deserve the punishment.
These words are not attributed to any specific person, but they are said as an indication that the entire universe will bear witness that they deserve what they are suffering, in accordance with the judgement of the Most Just, the All-Knowing.
Allah says:
قِيلَ ادْخُلُوا أَبْوَابَ جَهَنَّمَ خَالِدِينَ فِيهَا
(It will be said (to them): "Enter you the gates of Hell, to abide therein..."),
meaning, `to stay there forever; you will never leave or depart.'
...
فَبِيْسَ مَثْوَى الْمُتَكَبِّرِينَ
And (indeed) what an evil abode of the arrogant!
means, `what an evil destiny and what an evil place to stay, because of your arrogance in this world and your refusal to follow the truth; this is what has led you to this, what an evil state and what an evil final destination!'