The Sunni tradition fully confirms the event of Ghadir Khumm and no Sunni scholar to my knowledge has ever entirely denied it.
The Sunni masses, on the other hand, are largely unaware of this hugely important event. When known, it is considered one of many Prophetic merit narrations in favour of 'Ali and the other Companions and not anything specific to succession of 'Ali after the Prophet (a).
The differences come down to the details of what happened in that event and its context and, therefore, implication for the succession to the Prophet Muhammad (s).
You should first read the entire event here - https://www.al-islam.org/ghadir/incident.htm
The Shi'a believe that on the day of Ghadir Khumm, the Prophet Muhammad (s) was commanded to announce the system of his succession and introduce to his followers the first person who will immediately succeed him in that system.
So the system was declared through a segment of his speech that is famously referred to as the Hadith al-Thaqalayn. In that narration he stated that he was leaving behind the Qur'an and his Ahl al-Bayt, and if the people were to adhere to them both they would never go astray after him.
The person as well as the true nature of succession was introduced through the words man kuntu mawlahu fa 'Aliyyun mawlahu. It translates to:
- For whomsover I am master, 'Ali is his master.
The word used mawla was the perfect term to use for the comprehensive authority, walaya, that was possessed by the Prophet (s) and that was being vested in 'Ali as the first member of the chosen Ahl al-Bayt.
'Ali was not simply being made a Caliph nor just an Imam. Each of these labels are open to many interpretations of scope, geography, and timeframe. If designated a khalifa at Ghadir Khumm instead of mawla people could have accepted him as the first one but only for the years he ruled, and only for the area he ruled. If just called an imam, he could have been considered simply a prayer leader in a mosque!
People can get very creative when they want to derail a system, or if they have to explain away a derailed system from history.
So with Divine inspiration the Prophet (s) made clear the scope of the authority that was being vested in 'Ali in multiple ways. He repeated the portion of Qur'anic verse 33:6 as a question and got the public to acknowledge that he was closer to them and had more authority over them than their own souls. The man kuntu mawlahu designation itself tightly coupled his own walaya to that of 'Ali. And the Qur'anic verse 5:3 of ikmal that was revealed immediately thereafter announced the completion and perfection of religion as a result of that declaration.
Every single fact mentioned above is attested by Sunni sources of tafsir, hadith, and history. In all, 110 Companions of the Prophet (s) are documented to have narrated this event in varying levels of detail. And that makes Ghadir Khumm the most widely narrated hadith of the Prophet Muhammad (s) on any topic whatsoever in Sunni sources!
This is quite remarkable given that it was about an issue that divided the Muslim community. Its level of attestation despite that fact is certainly something to think about.
This raises the question of how the Sunni scholars understand the Ghadir Khumm event. Clearly, and by the very definition of being Sunni, they acknowledge the legitimacy of the caliphate of Abu Bakr. And so, for them, Ghadir Khumm was not about authority but about love and friendship towards 'Ali. This was based on taking an alternative meaning for mawla in the context of the event.
And for that context, it was considered to be unhappy soldiers from Yemen who had complained about 'Ali to the Prophet (s). But this theory is full of holes and cannot be reconciled with the full details of the event as documented on the link given earlier in this response.
As just one example, consider the fact that out of 110 Companions who narrated Ghadir Khumm, only 1 Companion - Burayda b. al-Husayb - mentions Yemen as the context. None of the other Companions mention Yemen or the unhappy soldiers as the context for the event.
And he was a Companion who was himself upset at 'Ali and was complaining to the Prophet (s) about him and thereby angered him (s) - all according to his own report!
There are many other holes in this Yemeni red herring narrative, feel free to ask another question about them and I can elaborate further, God-willing.