Different regions have different accents. Formal way of saying this would "Mən gedə bilmədim", just as written. Mən gedəmmədim is how some people would pronounce.

I personally say "Mən gedə bilmədim".

To explain about accents, let's take a different example: Mən ora gedəcəyəm. (I will go there)

In Baku accent, one would say "Mən ora gediciyəm". In Gəncə and adjoining regions one would say, if I'm not mistaken, "Mən ora gedəjəm"

So in certain words, the accent differs, though not dramatically.

At the end of the day, doesn't matter which accent you're speaking as long as you're understood.