'The Guru Has Given me the Understanding of Oneness [Advait]' - Jap Ji Sahib

The Great poet Santhok Singh, composer of the epic Gurpratap Suraj Prakash Granth

The prolific poet and historian, Kavi Santhōk Singh [1787-1843] was not only a historian but also a great scholar. Several years before writing the enormous historical book, Gurpratāp Sūraj Prakāsh Granth, Kavi Ji was commissioned by Maharājā Udai Singh in 1829 to write a commentary on Jap Ji Sāhib. Kavi Ji's deep knowledge of Vedāntā is strikingly apparent in his interpretation of the end of the 6th verse in Jap Ji Sahib [shown below].

ਗੁਰਾ ਇਕ ਦੇਹਿ ਬੁਝਾਈ ॥

ਸਚਿਦਾਨੰਦ ਕਾਹੇ ਤੇ ਗੁਰ ਦੇਤ ਹੈਂ, ਸੋਈ ਬਤਾਵਤ ਹੈਂ । ਅਦ੍ਵੈਤ ਹੈ ਏਕ ਹੀ ਪੂਰਣ ਸਰਬ ਮੈ ਇਸ ਬੂਝ ਕੋ ਸਤਿਗੁਰ ਬੁਝਾਇ ਦੇਤ ਹੈਂ ।
The Guru here explains the concept [of Vahiguru] being ਸਚਿਦਾਨੰਦ  [Everlasting-Consciousness-of Bliss]. [Vahiguru is] Advait [non-dual], and that One that is everything I am also All of that, this is the understanding that Satguru has given.

ਸਭਨਾ ਜੀਆ ਕਾ ਇਕੁ ਦਾਤਾ, ਸੋ ਮੈ, ਵਿਸਰਿ ਨ ਜਾਈ ॥

ਸਬਰ ਜੀਅਨ ਕੋ ਜੋ ਏਕ ਦਾਤਾ ਹੈ ਸੋ ਮੈਂ, ਕਿਆ ? ਸੋਹੰ, ਇਹ ਬੂਝ ਸਤਿਗੁਰੋਂ ਨੇ ਦਈ, ਸੋ ਬਿਸਰੈ ਕਦਾਚਿਤ ਨਹੀਂ ।
All of the Beings  have One Giver, and I am That. How? The Guru has given the understanding of ਸੋਹੰ [I am That/Vahiguru]. May I never ever forget this [teaching]. 
Garab Ganjani Tika, page 53-54