How is catharsis different to gossiping in Buddhism? Catharsis is described as "providing psychological relief through the open expression of strong emotions; causing catharsis"

An online resource mentions "The Buddha has thus presented the principles of the catharsis of emotions..."

It is my understanding that gossip is not looked upon favorably in Buddhism. I am unsure why though. 

Rightly or wrongly, i practice catharsis through open dialogue with others. This may be in relation to others or myself. For example, if i am faced with a challenge which i am aware and mindful gives rise to emotions such as frustration, anger, helplessness, etc, i often discuss these with close family. 

Working with a real-world example, currently i am faced with someone who simply does not wish to work with others in a meaningful way nor takes proactive steps to work with others. Often this is seen as being a roadblock of sorts and more than often, there is little trust in working with the person. I have attempted on multiple occasions to work with this person in a way to go beyond the trifles and historical 'negative' experiences which has been less than successful. Now i undertake catharsis by discussing the concept of 'how it blows my mind" that someone is not willing to work in meaningful ways.
A couple of reasons why your practice of catharsis, as described above, might not be considered a favorable thing to do in Buddhism even if you believe it's different enough from gossip to not be considered wrong speech. 

It brings you out of the present moment and has you re-living or clinging to the past.


  Let one not trace back the past
  
  Or yearn for the future-yet-to-come.
  
  That which is past is left behind
  
  Unattained is the "yet-to-come."
  
  But that which is present he discerns —
  
  With insight as and when it comes.
  
  The Immovable — the-non-irritable.
  
  In that state should the wise one grow
  
  Today itself should one bestir
  
  Tomorrow death may come — who knows?
  
  For no bargain can we strike
  
  With Death who has his mighty hosts.
  
  But one who dwells thus ardently
  
  By day, by night, untiringly
  
  Him the Tranquil Sage has called
  
  The Ideal Lover of Solitude.
  
  MN 131


Also, it's the opposite of Equanimity
which is prized as the 4th brahmavihāra, the 7th Factor of Enlightenment, and the 10th pāramī.

Lastly, spending time blowing off steam about another person comes close to gossip, ridicule, and other types of speech which are not considered Right Speech and could easily cross the line. As Anthony suggested above, reading up on Right Speech might help to make sense of all this. Best wishes.