Gnosis is an old Greek word. It means knowledge, but a particular kind. Not knowledge from a book or a teacher. Knowledge from experience.
Reading about the sea is not the same as swimming in it. You can learn every fact about salt water and still not know what it is like to be lifted by a wave. The person who has swum knows something the reader cannot, however much the reader studies.
Every great tradition holds both kinds of knowing. There are its teachings, which can be read, and its experiences, which have to be lived. Over the centuries the teachings get written down and argued over. The experiences get forgotten. Gnosis is the name for the experiences, and for the way back to them.
So this book asks very little belief of you. It describes something, and then asks you to look. Where it says your attention wanders, watch it wander. Where it speaks of the night, try the practice and see what happens.
A teaching you have tested becomes yours. A teaching you only believe still belongs to someone else.