The /ð/ sound is spelled with <th>. The /ŋ/ sound is spelled with <ng>.
This is a contrast between a dental fricative and a velar nasal continuant, both voiced. It can only occur medially or finally. Although the individual sounds may not occur or may have restricted environments in many languages, the contrast does not cause problems.
Since /ŋ/ is a fairly common sound while /ð is rare, the mean density value is low at 0.1%. The list makes 4 semantic distinctions, a loading of 57%.
gather ganger gathers gangers slither slinger slithers slingers whither winger withers wingers with wing
John Higgins, Shaftesbury, November 2010.