In Greek, γγ has long been pronounced ng. I do not know the exact origin of this and, to top it, Greek pronounciation has undergone major changes (stops have become fricatives in non-nasal contexts), and lost much of the dialectical variations, so that modern pronounciation is not a good guide to many phenomenon. But, at least in the gamma-gamma and the gamma-chi contexts, the first gamma is a nasal sound, and in gamma-kappa it is a non-nasal stop, not a fricative. (More understandably, a preceding nasal often merely marks a modern fricative as a stop.)
In any case, at least today, it is an orthographic issue: and Evaggelos is a transliteration which captures the orthography and *not* the pronounciation, Evangelos does the opposite. I do not know where the use of ni-gamma appeared in the few examples on Google: whether they are mistakes or regional orthography.