"I speak English."
Greek spelling is pretty good at giving you the pronunciation once you learn the symbol-to-sound mappings.
The only major ambiguity I can think of is μπ ντ γκ which can be /b d g/ or /mb nd ŋg/ or in some cases /mp nt ŋk/, but I don't think that the distinction can ever create a distinct word.
It's the other way around that's difficult sometimes: the same sound can be written in multiple ways (e.g. η οι ει υι ι υ for /i/, αυ αβ for /av/).
Excuse me but do you speak modern Greek or ancient Greek? In modern, "γγ" actually is pronounced as "γκ", so it's like "g" in words such as "αγγούρι" or "αγγλικά" (sounds like the "g" in "grammar") But there are exceptions where you pronounce it as it is only one "γ", like "συγγνώμη" (you pronounce it "συγνώμη") or other exceptions where you pronounce the "γγ" as "νγ" as you said. Like the word "έγγραφο" which sounds like "ένγραφο" when you speak.