What is the truth that you deny?
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