Rabban Yochanan ben Zakai expressed to his students his uncertainty regarding “on which path they will take me – Gehinom or Gan Eden”. However at the very end the Gemara relates that he exclaimed “prepare a chair for Chizkiyahu who has come to escort me” which apparently indicates he was on the way to Gan Eden!
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In the artscroll notes note 19 they have brought down from the Maharsha
He felt That even though he wont be totally destroyed he might pass by Gehinom for a limited time to purge any of his sins before he enters in Gan Eden
Sdeh Tzofim brings down a Zohar PArshas Vayechi that when a person passes away reshus is given to him to see his relatives and friends tin the nexto world. If he is a tzaddik, they accompany him on his journey to Gan Eden. Presumable Rabban Yochanan ben Zakkai experienced this in his last moments.