It's a guess, as I was also never shown a "Ng Mun" form, but Hung Ga has "Sei Mun" (4 Gates), which basically covers 4 directions of attack/defense.
It's beginner stuff but very important to quite a few styles, and especially important for san da training. The gates/bridges system is found all over, you need to learn those principles in some form.
Dare I ask, is there video somewhere of Ng Mun or is that the unicorn? If Ng Mun does mean "5 Gates", there's got to be a list somewhere...
There are several videos online of the form. None of Bruce himself doing the form as he tended to showcase those forms that were visually impressive to land his roles. But yes, the form does mean 5 gates and its (edit: PRINCIPLES) borrowed from WC. Other styles refer to them differently, but since WC is primarily CQC, the gates refer to 2upper, 2lower, and 1base. I also learned the same in WC, validating the origin, though different Sifu interpretations may vary. Some say 4, some 6, some 6 + in or out for 12. etc etc.
Philosophically (from his own notes) this is what Bruce meant by having no form, your forms grow to infinity as you unlock your personal potential. For me personally, that did happen somewhere around my middle years when I'd learned a bunch of styles and recognized what they had in common...core athletic principles, agility, timing, resistance, and so on.
This is an interesting take, and I respect it... but my experience has been quite the opposite to this description. May be a topic for another thread but having "No Form" from my vantage point, stems from Daoist/Do-ist philosophy as we know and means to reduce/refine ourselves in various ways, but moreso to reduce our movements in specific relation to physical. Once we have reached a proficient base level, reducing preparatory movement, reducing recovery movements, reducing variety of reflexive responses to causes, reducing labels and distinction between techniques, all are examples of being in sync with Dao/Do. In a tangible example to help explain, understanding that the only true characteristic difference between a hook and an upper is degree of tilt. So these techs are thus situational and circumstantially inferred by the target/opponent, therefore everything from hook to upper and every degree of angle in between is essentially the same. So "I" don't need to concern myself with right or wrong tech selection, or hook or upper labels, or different interpretations of the two techs from various styles etc. etc.... but rather just respond to a given cause when it presents itself. We have to train to be present in the moment to do this effectively. And that may be simplifying some things a bit, but it is the jist.
It's the 'Do' in JKD and actively maintains the principle of Do/Daoism. Those who study Do/Daoism will find the principle is also analogous to other tenets of Doism, like "mo-sam" or the mind of no mind, "mo wai" or the action of no-action. etc. etc. Quite fitting for the way of no-way, or the style of no-style, or form of no-form in my opinion. Unfortunately not enough of us are intrigued enough to investigate Do/Daoism that the style is based on, and are instead scared off from the possibility of dogmatic religious doctrine.