There are three bands named Gehenna.
The first Gehenna (Norway) was formed in January 1993 as a nameless project by Sanrabb, Dolgar and Sir Vereda. They didn't give the band a name until they had enough material which would represent their voice. In May of the same year, the trio entered the studio to record a Demo. Sanrabb and Dolgar shared the duty of guitars, vocals, bass and synth, while Sir Vereda took his seat behind the drum set.
The demo was recorded between the 3rd and 7th of June 1993 in Soundsuite Studios. It was released under The name "Black Seared Heart" as a tape limited to 100 copies.

After several years of inactivity, the band returned with WW in 2005, heralding a much welcomed return to their black metal roots.

The following is a far more accurate biography taken from allmusic.com:

According to some gospels, when Jesus Christ referred to Hell, he often used the word Gehenna, which was actually the name of Jerusalem's garbage dump -- where fires were constantly kept burning to cope with the city's refuse. Fast forward roughly 2,000 years and the name was adopted by one of Norway's most influential black metal bands, which formed in the town of Stavanger, in January 1993, in the wake of local forefathers Mayhem's ascendance, and integrated a now heralded generation also including such contemporaries as Emperor, Enslaved, Darkthrone, and Ulver. Initially comprising vocalist/guitarist Sanrabb, guitarist/bassist Dolgar (real name Steffen Simenstad), and drummer Sir Vereda, the initially nameless project that would become Gehenna recorded a pair of demos, Black Seared Heart and Ancestors of the Darkly Sky, in their first year, before earning their Scandinavian black metal stripes in dubious fashion when the drummer joined several other more prominent scene members behind bars. Even so, by the following year, Sanrabb and Dolgar had drafted bassist Svartalv, keyboardist Sarcana, and drummer Dirge Rep (aka Per Husebø), played their first local shows, and recorded a debut mini-album entitled First Spell for Head Not Found Records. Still widely regarded to be their definitive work, First Spell showcased a unique style of black metal, both somber and melodic in nature, and earned the group a new recording contract with Cacophonous Records, which also released subsequent efforts Seen Through the Veils of Darkness (1995) and Malice (1996) to serious acclaim in the underground metal world. But Gehenna almost came to a grinding halt in 1997, when internal tensions led to Svartalv's departure to found thrash band Nocturnal Breed, Dirge Rep's defection to Enslaved, and Sarcana's retirement from the scene. Sanrabb and Dolgar would re-emerge the following year with a new album entitled Adimiron Black for Moonfog Productions, and new members E.N. Death (bass), Blod (drums), and Damien (keyboards), but the latter would soon be dismissed due to Gehenna's distinctly more aggressive sound. In fact, by the release of 2000's Murder, Gehenna had reinvented themselves as a death metal band, leaving many observers puzzled. Come 2005's WW, they'd once again returned to their haunting black metal roots, but time has yet to tell whether their fans returned to it with them.

The other Gehenna is a nihilistic metallic hardcore band from San Diego, CA. / Reno, NV. one of hardcore's most notorious and vicious bands, thats have released records on CrimethInc, Crawlspace and Area 51.

Third band is Progressive Heavy Metal band from Mexico.Back in 1987 this band released one LP called "EN BUSCA DEL VALLE DE GEHENNA"

Also note, there is a Swedish Metal band called Gehennah. They play an old style of Thrash. Note; they spell their name with an "h" on the end.