The mishna in Sanhedrin 90a lists Gehazi as one of the 4 commoners who have no share in the world to come. I have seen different reasons for this none of which I fully understand.
He mocked the words of the sages - artscroll says this, not sure when he did this but its not included as one of the reasons why someone would lose their olam haba in the mishna
he denied the resurrection - this is quite possible according to the midrashim that say he spoke (disparagingly) on his way to revive the shunamite woman's son. But later on he extols his master's ability to resurrect that same child to the king. So even if he may have doubted in the past he clearly no longer does.
He pronounced the name of God - this is my own interpretation which I haven't seen elsewhere. The mishna lists that as one of the reasons to forfeit olam haba [according to one opinion] and Gehazi seems to have done something very similar to make Yerovam's calf speak (see Sotah 47a)
He didn't repent for causing others to sin - found in otzar aggados - similarly not listed in the mishna.
I'm looking for a clear-cut reason why Gehazi has no share in the world to come.