It is written at many places in Gurbani. First, we take from Japjisahib, where in lst pauri, Guru NANAK raises a million dollar question 'keev sachiara hoeye keev kudhe teuteh pal -Guru nanak answers this question in fourth pauri 'amrit velya sach nao vadhaaiye veeechar, Good thoughts, remembering His name in the early morning and singing His song in order to churn Naam i.e. Gur sabd. Then Guru Nanak clarifies, 'nanak aavai janeiye sabh aapai sachiar - that if we remember and mediate upon Him alongwith being good human by reflecting good thoughts of gurbani, I stand guarantee that you will be truthful and or able to illuminate the jyot inside. Since Nanak is mentioned in this pankti it stands Guru Nanak himself is witnessing and guaranteeing.Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/gurmat-vichaar/17651-why-do-we-pray.html
I feel you are misinterpretting Guru waak"NANAK EVAI JANIAI SABH AAPE SACHYAAR" jAPJI (4)Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=17651
It should be," Nanak says this is the way( when early hours with fresh mind we meditate on Him) it is learnt that The External Waheguru himself is every thing and permeates in everywhere"
That guarantee part appears out of context.