The question is just the title. Moreover, AFAIK, currently there are 2 editions of the 14th Dalai Lama Guru Yoga.
(1) "The Source of All Attainments: The Yoga of the Inseparability of the Guru and Avalokiteshvara".
(2) "A Stairway to Potala: Guru Yoga of Avalokiteshvara".
If practiced properly from a proper teacher, it is:
The fastest way to attain enlightenment.
Some of the reasons are:
- The guru is the Buddha
- if the guru is the Buddha we have direct access to Buddha teaching
this because: a method was developed by the Buddha, it was transmitted Orally for the first 500 years, after that, books where introduced with the Buddhas teachings to supplement the oral teachings. This teachings are still being teach verbally and in other ways as well. Qualified teachers (Gurus) have complete understanding from where the teachings came (lineage) and there is still unbroken lineages around.
IF we choose to practice Guru yoga. It is recommended that we do our homework and find a proper teacher(s) that we can trust and we feel connected. Also absolutely proper to ask question about lineage and from who they learn. Guru Yoga and Vajrayana wont work if is from a not proper teacher and is said to be dangerous if practice wrongly.
I hope this answer the question! also sorry if there is any typo.