What is the truth that you deny? I have heard that there is a Tibetan word that roughly translates to "The truth that we deny" and sounded close to the English word "gotcha."

I however cannot locate this word or a source of it. Can anyone provide a source?

It is possible that it translates to either "The truth that I deny" or "The truth that you deny."
I am not sure if "we" was the right pronoun. 
I think it might have been used in debates within the monasteries.
The word, as spelled in english, is "gak-cha," and it refers to the object whose existence we deny.  

The idea is that things, including our selves, do not exist "out there, independently," or "from their own side."  Such a thing, according to Mahayana buddhism, could never exist.  When we catch our mind falling prey to relating to some thing (including our "self") as if it really existed independently from our perceiving mind, then we have located the "gak-cha."

There's a transcription of a talk by Lama Yeshe that goes into some depth:
Lama Yeshe Archive