While Kenny is travelling to hell in South Park: Bigger, Longer & Uncut, he is welcomed by personalities including Gandhi.
Why did South Park make fun of Mahatma Gandhi by showing him in hell?
South Park is a comedy animated show and they throw satire on everything they can. Here is a list of subjects they made fun about.
For this matter, many religions and sects believe that only with their religions people will go to heaven etc. Many Christian believers also thinks this way. South Park just made fun of it.
In the South Park Universe, only Mormons are sent to Heaven after their death, so basically everyone in history who wasn't a Mormon during their lives is currently living in Hell; including Hitler, Gandhi, JFK, Jeffrey Dahmer, Gene Siskel, and many many more. - southpark.wikia.com.
There was no much deep thoughts about Gandhi specifically and South Park is just being South Park.
It's also clear from one of the episode too: