"knowing" in this case acts like an infinitive: you can read it like: "to know". It acts as the subject of the main clause, that is: "knowing (meaning: the fact of knowing)...is far more important to me than money".
When an infinitive acts as a subject and it's at the beginning of a sentence, the English language prefers using an -ing form.
There is an understood "that" which introduces a subordinate clause: "...that I'm helping others...".