I did some Chinese courses last year and "na ge" is something that will get repeated frequently because of how Chinese make their sentences. There is another one "nei ge". They are an important part of the Chinese language.
Edit: just watched the video. He was not teaching a chinese course. A bit of a suspicious example he picked if you ask me.
On the other hand, if you're an African American doing a business deal in China, it is also almost certainly the best example, and might genuinely be the most valuable thing that you take away from that class. Specifically, "sometimes people speaking Mandarin say something that sounds like 'ni__er'. It is not actually 'ni__er', and is completely unrelated to my race."
That could easily be the difference between "deal" and "end of relationship".
The filler word is "nei ge" and not "neh gah". The "ge" is pronounced as "gue" and "nei" like "ney". But it might have been a totally innocent mistake.
Edit: just watched the video. He was not teaching a chinese course. A bit of a suspicious example he picked if you ask me.