This question is complicated by the fact that many Bible versions translate both Gehenna and Hades as the English word "Hell".
Online Etymology Dictionary says this of the original derivation of the word "hell":
Literally "concealed place" (compare Old Norse hellir "cave, cavern"), from PIE root *kel- (1) "to cover, conceal, save."
Storing potatoes in a cellar over winter used to be known as "helling potatoes".
But today we popularly associate that word with a supernatural world, popularized by Dante's Divine Comedy.
This idea of a place where unsaved immortal souls are tortured forever and ever is a central doctrine of both Roman Catholicism and many Protestant denominations.
But Biblically, there is very little support for such a concept.
And the underlying idea behind it fundamentally goes against the Biblical view of God as being loving and forgiving.
In order to understand the difference between Gehenna and Hades, we must avoid the word "Hell" and even more importantly avoid letting any thoughts about the popular view of Hell enter into the analysis.
The Hebrew scriptures have two words with similar meanings:
- "qeber" refers to a tomb, sepulchre, burying place, grave site, memorial, etc.
It is the physical place or evidence of burial that living people can see and visit.
- "sheol" is the hidden place where a body actually is.
It can't be seen without exhumation, etc.
Neither word necessarily has any supernatural meaning.
Similarly, the Greek scriptures have two words with similar meanings:
- "Hades" (ᾅδης) is the same as "sheol". It is simply the grave where bodies are buried and decompose.
- "Gehenna" (γέεννα) refers to a garbage dump where trash and unwanted bodies are burned to ashes.
"Hades" signifies physical death, awaiting resurrection.
"Gehenna" signifies total destruction and nothingness.
Neither of them imply conscious awareness or being tortured for ever, as popularized by Dante.
Throughout the Greek scriptures, "Hades" simply refers to the physical grave where a body is buried.
It has no supernatural meaning of itself, but is sometimes used to symbolize unconscious dead people awaiting resurrection.
"Gehenna" is used more metaphorically, to indicate total and permanent destruction.
Many scriptures, especially the book of Revelation, talk of both concepts: those that "sleep" in Hades and will one day be resurrected; and those that will be burned to ashes.
Eventually all humanity will be resurrected (in one of three general resurrections).
Those that are saved will be given immortality, while those that choose to remain incorrigible will be destroyed.
The concept of an "immortal soul" appears nowhere in the Bible.
In fact, quite the opposite. As Ezekiel 18:20 says: "The soul that sinneth, it shall die.".