The 'g' is soft: softer than 'jam', more like the second 's' in persuasion.
The first vowel is not a flat a. Because it's followed by an "n" it's pronunciation is somewhere in between the English "munge", "horn", "harm".
In French the vowels change their pronunciation when followed by 'n': so "an" and "en", "in", "on", and "un" have a specific pronunciation.
The final vowel "e" in "mange" is silent (unless it has an accent like "mangé"): "mange" is a one-syllable word.
It's similar to the English word "munge" except that in English you put the tip of your tongue on the roof of your mouth to pronounce the "n" and in French you don't: in French that "n" is mixed into the pronunciation of the preceding vowel and (like any vowel) is pronounced with the shape of the mouth rather than with the tip of the tongue: you should think of "an" as being a single letter for the purposes of pronunciation.
French doesn't seem to be pronounced as it is written.
Actually I think it is, i.e. (unlike English) you can guess the pronunciation from the spelling.
There's a trick to it though:
- The pronunciation of vowels is changed by accents
You often need to consider the pronunciation of pairs of letters, for example:
- g and c are 'hard' in front of a, o, and u; and are soft in front of e and i
- each vowel has a different pronunciation when followed by n or m
- maybe a few other (mostly learnable or regular) rules like that
Often letters (especially at the end of a word) are silent (not pronounced at all), and visibly present only in the spelling, to hint at the grammar or the etymology of the word