Usually we pronounce "ariGato" and "ikaGa" . That's similar to IPA [g] sound , but Japanese "g" sound is weaker than English "g" sound. Some Japanese would pronounce IPA [ŋ] sound , like "ariŋato" and "ikaŋa" We don't pronounce "ariNGato" and "ikaNGa" in any situation. I'm not a professional phonetician , that's just my opinion. I hope this will help.

Hi! (arigatou) and (ikaga) are pronounced the way they are. No (n) exists before (g). I think the case you're referring to is pronouncing words containing (n), succeeded by (g) --> "ng". For example: (gengo; gen + go) "language", (tango; tan + go) "word", (manga; man + ga) "Manga". In some other cases, when (n) is succeeded by (k), it's pronounced (ng), as in (tengoku; ten + koku) "heaven".