Got me thinking if it’s known that the person has gone to Gan Eden does those who say Kaddish need to continue saying it?
Given that we say kaddish on the yahrtzeit even though the neshama is assumed to be in Gan Eden, we should say kaddish for the first 11 months in any case. We would be saying kaddish so that the neshama should have an aliyah no matter how small any aveiros might be or even if the neshama immediately is placed at an initial level in Gan Eden. I am assuming that this is a parent that you are saying kaddish for as anyone else would not require you to say kaddish at all.
First and foremost we do not rely on dreams since many times they are nonsensical. In fact ,the Noda B'Yehuda writes in Yoreh Deah Tinyana siman 30 he notes that dreams are not reliable and writes " where do we ever see halacha learned from a dream!?!?".further in the teshuva he writes " to bring a proof from a dream is nonsense and nothing at all".
Besides for the reason above one of the reason why the kaadish was instituted for a mourner was to show that the family accepts the loss of their family member by sanctifying HaShems name in a public forum,and show that they still believe in G-d.
While dreams "can" be prophetic, the likelihood of them being so is low for the average person. The reality is we aren't supposed to take dreams this seriously as we're advised the majority of our dreams are either nonsense, reflections of the previous day, or expressing issues within ourselves.
There is actually a bit discussed about the topic in the Talmud. The readings themselves are quite interesting but they get touched upon in this article found on Aish.
The general consensus is we should assume a dream is just a dream.
A prophetic dream can certainly exist but we can't actually know it's a prophecy until it happens. This creates a predicament in your scenario.
If you say Kaddish, you're recognizing your own fallibility as a person. You aren't assuming prophecy but assuming you're human and doing what all humans should do. You say Kaddish because you can't actually know for sure.
If you don't say Kaddish and assume the dream was true, you're making an assumption which you have no true proof of. The dream may have made you feel secure in the idea your loved one is in a better place but you don't actually know for sure. You then use that assumption to ignore the responsibility of saying Kaddish.
Dreams are odd because they can be related to a mix of things.
The food you ate before bed.
Anticipation of an upcoming good/bad event.
Sickness fueling odd dreams (a fever dream).
You would be advised to say Kaddish because you can't actually know. If the dream wasn't prophetic and simply created through a subconscious means or external stimuli, you would be ignoring the need to say Kaddish. That isn't an assumption worth making.
The source for saying Kaddish for the deceased is from Kalla Rabbasi 2:9 (as mentioned by the Darchei Moshe Y.D. 376:8):
ט ברייתא קטנים מקבלים פני שכינה שנאמר זרע יעבדנו יסופר לה׳ לדור: [גמ׳] איבעיא להו מכפרין עון אבות או לא, ת״ש דר׳ עקיבא נפק לההוא אתרא אשכחיה לההוא גברא דהוי דרי טונא אכתפיה ולא הוה מצי לסגויי ביה והוה צוח ומתאנח, א״ל מאי עבידתיך א״ל לא שבקנא איסורא דלא עבידנא בההיא עלמא ועכשיו איכא נטורין עילוון ולא שבקין לי דאינוח א״ל רבי עקיבא שבקת ברא א״ל בחייך לא תשלין דדחילנא ממלאכי דמחו לי בפולסי דנורא ואמרין לי אמאי לא תיתי בפריע א״ל אימא ליה דקא ניחותך, א״ל שבקית אתתא מעברתא אזל ר׳ עקיבא עאל לההיא מדינתא, אמר להו בריה דפלוני היכא (ליה) אמרו ליה יעקר זכרו דההוא שחיק עצמות א״ל אמאי אמרו ליה ההוא לסטים אכל אינשי ומצער בריתא ולא עוד אלא שבא על נערה המאורסה ביום הכפורים. אזל לביתיה אשכח אתתיה מעוברתא נטרה עד דילדה , אזל מהליה לכי גדל אוקמי׳ בבי כנישתא לברוכי בקהלא לימים אזל ר׳ עקיבא לההוא אתרא איתחזי ליה א״ל תנוח דעתך שהנחת את דעתי:
To summarize, Rebbi Akiva saw a dead man suffering from his punishment, and he wanted to save the dead man from his suffering. To do that he found the dead man's son, and had him say berachos for the congregation. Rebbi Akiva later found that the dead man was settled.
The Beis Yosef brings a similar story from the Zohar where the son says Kaddish or the Haftarah.
According to this the whole purpose of saying Kaddish is to bring the deceased to Gan Eden, and if one were to know that he is already there , then there would be no point.