The thing to remember is that there are at least two ways to say everything in Korean - a polite way and an informal way used among friends and family. The Korean phrase for the polite way, "No-ppun Mal" literally translates as "High language".
In the above write-up, "Goh mabb subb nee dah" and "Goh mah wuh" (which should be Goh mabb sum-nee-dah - the b/p sound at the end of the third syllable is alliterated into an 'm' sound) are the same exact words, with the first having the polite ending and the second having the informal ending. The first phrase given, "Gam sah ham nee dah" could be said informally as "Gam sah hay", but usually is not, by convention only, not for grammatical reasons.
While this may seem confusing to have two ways to say everything, it is no harder to learn that the gender of nouns is necessary when speaking French, Spanish, etc.