One Upanishad, that is very close to the essence of Gurbani, is Kenopanishad. One of its first pronouncements is, "सर्वं ब्रह्मौपनिषदं" - "Brahman is all of which the Upanishads speak."
Naming of Kenopanishad is not different from something like Japji Sahib, or Sodar -- where the first few word/s of a poem are used to name the rest of the poem.
In this case, Ken comes from Kena (Sanskrit: केन) which literally means, depending on the object-subject context, "by what, by whom, whence, how, why, from what cause". This root of Kena, in the sense of "by whom" or "from what cause", is found the inquisitive first verse of the Kena Upanishad as follows,
केनेषितं पतति प्रेषितं मनः
केन प्राणः प्रथमः प्रैति युक्तः ।
केनेषितां वाचमिमां वदन्ति
चक्षुः श्रोत्रं क उ देवो युनक्ति ॥ १ ॥
Sent by whom, flies out thither the mind?
Harnessed by whom, roves thither the first breath?
Who sends out the speech which we speak?
Who is the Deva (deity, god) that harnesses the ears and eyes?
—Kena Upanishad 1.1 —Translated by Paul Deussen