There are two issues: (1) heuristics used in phonics (2) phonetics
g: /g/, /dʒ/
c: /k/, /s/
/k/ is voiceless, whereas its counterpart /g/ is voiced.
/s/ is voicelss; /dʒ/ is voiced.
Velar consonants get softened (or voiced) before front vowels esp in derived-words: cf. electric vs electricity
Given this background knowledge, what is your concern?