Is there a rule for the pronunciation of G and CH in German?
The G in TAG and ZWANZIG are pronounced differently, the same happens with CH in ICH and BUCH: How do I know if my pronunciation is right?

Question

Answer

The rules for 'ch' are summed up here: http://de.wikipedia.org/wiki/Ch_(Digraph)

For 'g' generally the "hard" pronunciation is correct, like in 'Tag'. There's only an exception for words ending in '-ig', more about that here: http://www.gfds.de/sprachberatung/fragen-und-antworten/uebersichtsseite/aussprache-von-ig/