The Poker Face hitmaker urged the reality TV star to "set a standard of respect, compassion, and love" and "take responsibility and accountability" for her catty comments on US style series Fashion Police.
The letter, posted on Gaga's LittleMonsters.com blog, came after Kelly revealed she had been cyber bullied by the hitmaker's fans for harsh comments she made about the pop star's weight.
Gaga wrote: "I have empathy for you Kelly, but I feel it culturally important to note that you have chosen a less compassionate path. Your work... with the Fashion Police is rooted in criticism, judgment, and rating people's beauty against one another. 'Appearance' is the most used reason for bullying in the world.
"It used to make me truly sad when I would hear people talk about your weight when you were younger, as I was bullied too. To see you blossoming into a beautiful slender woman who makes fun of others for a living is astounding."
But Gaga's note has not gone down well with Sharon, who has defended Kelly in a lengthy post on her Facebook page.
The former America's Got Talent judge slams Gaga for her decision to wear fur and insists she was "sickened" by the raw meat outfit she wore at the 2010 MTV Video Music Awards.
She rages: "Your open letter is hypocritical and full of contradictions. How sweet that you have empathy for my daughter, as you feel that she has taken a less passionate path in life. You say her work on... Fashion Police is 'rooted in criticism, judgment, and rating people's beauty against one another.' Welcome to the real world.
"Example, when I saw you wear a dress made out of raw meat, I was sickened. When I see you wearing fur, and using it as a fashion statement, the fact that defenseless animals have been killed so you can get your picture in the press is abhorrent to me. Shouldn't you be teaching your 'little monster' fans to respect animals and life?
"By your actions to Kelly right now, you have shown me that you are nothing more than a publicly seeking hypocrite and an attention seeker. You know it would have been much more dignified of you to do this privately. I am calling you a bully because you have 32 million followers hanging on your every word and you are criticizing Kelly in your open letter.
"Stop wearing fur, stop looking for publicity, and stop using your fans to belittle not just Kelly but an endless stream of celebrities. A word from you would stop all the hideous, negative and vile threats from your 'little monsters.' Let me know if you want to continue this debate. I'm an open playing field for you my darling."
Kelly also addressed the drama in a post on her Twitter page, adding: "I hope you guys realize that I don't care about any of this bulls***! I know who I am I don't need anyone to tell me!... love to all."