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.
gather ganger gathers gangers slither slinger slithers slingers whither winger withers wingers with wing