The spelling of /g/ is <g>. The /ʒ/ sound is spelled <su>.
The contrast is between a stop and a fricative, both voiced but well separated in the mouth. It is unlikely to cause a problem.
The /g/ sound can occur in any position in the syllable, but /ʒ/ only occurs finally or intervocalically, other than in foreign names or loan words such as gendarme. /ʒ/ is in any case a very rare phoneme, found in only 334 words in the source dictionary.
The figure for the mean density is too small to be worth calculating. With only one entry in the set the semantic loading is 100%.
John Higgins, Shaftesbury, January 2010.