From the purport to Madhya 8.138
As guest pointed pointed out from Lord Brahma:
"By the recollection of kama-gayatri it seems to me that I am the eternal maidservant of Krsna." Though the other mysteries in regard to the condition of the maidservant of Krsna were not revealed to him, Brahma, by dint of his searching self-consciousness, became well acquainted with the ocean of truth. All the truths of the Vedas were revealed to him"
The Kama Gayatri tells us a limited amount of information. Brahma was awakened towards the highest reality by Kama Gayatri, but he was unable to gain entrance into the mysteries of Krishna by that. It was Brahma's "searching self consciousness" which was to lead to the mysteries of Sri Krishna being revealed to him.
So my original question was:
It appears that the knowledge that the mantra gives is limited, it is an introduction to the highest truth. A mantra of 24 syllables is limited in the amount of knowledge it can give. The words of the Kama Gayatri point one towards madhurya rasa, by hearing that mantra Brahma became awakened towards the ultimate truth of the Vedas final conclusion. In order for one to become well acquainted with the highest truth there must be a much deeper level of self realization and acquisition of knowledge.
So I still have to wonder how a mantra can give:
"all kinds of divya-jnana, transcendental knowledge, including the knowledge of krsna-tattva and jiva-tattva, and it also gives a relationship with Krsna"
It would appear to me that the amount of divya jnana mentioned above seems to be something which is not given by any mantra. Brahma had to search within himself for some time to become well acquainted with the ocean of truth after reception of the gayatri. We are similar to Brahma in that although a mantra can awaken us to the highest truth to some degree, it will take searching within to have the highest truth revealed to us.
We can see this to be true from our own experience. How many of us and how many people do we know who have been initiated into chanting Gayatri mantras, the mahamantra, or any number of mantras? How many of those are actually self realized souls due to chanting those mantras for years on end? How many have entered into a direct relationship with Sri Radha Krishna due to those mantras?
Ultimately the mantra is a tool to aide us in our progression towards self realization. While the mantra is non different from Sri Krishna, it's purpose is to inspire us to bring us closer to Krishna. For that closeness to actually occur we have to search within ourselves, it is from within where we will find the highest truth revealed to us. It is within our selves where we will find Sri Radha Krishna ready to reveal all. The mantra inspires us and purifies us, but the actual knowledge of the highest truth and our relationship with it is something we can only experience by awakening to what lies at the core of our being. We need to look inward, not within our bodies, within our minds, our souls, it is there where we will find God as our constant companion. That is where the final truth is found. Only from there can we know who we truly are and what our relationship with God truly is and can be.