i'm not sure how to answer this but in my humble opinion Gurmat is that which is sanctioned and taught by your Guru.
essentially, our mind is covered with a curtain of maya and the five vices. when we use it to do things, we sometimes act from the perspective of maya and the five vices.
however, if we only use the guru's teachings as a reference point for how to act in our life - then we will be embracing and following gurmat.
"man kee mat mohay sagal teaagee"
it is important to "tyaag" or abandon manmat - and adopt guru's mat
Guru's mat is guru's intellect, and in order to have guru's intellect we need to read guru's bani.
Guru's bani lets us know what is right and wrong.
And since I don't read much, I'm not quite sure what's right and waht's wrong at this time... hehehe... so I'll shut up
bhul chuk maaf karnaa jeeo