"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...".

It is really helpful for me because ı face with the sentences like tis for a lot of time. Thank you so much!

There is a hidden "that" after the gerund "Knowing". Most of the time, native English speakers omit saying "that" in an informal setting. You can always add this word when you want to speak more formally. "Knowing" in this context means that the knowledge of being able to help others is more fulfilling than being able to earn money.