The affix khañ comes in the sense of 'it formerly had been' after the word goṣṭha,
The word गोष्ठ is a compound of गो + स्था i.e. a cow-pen, it means the place all around a cow-pen. The word भूतपूर्व qualifies it. Thus गोष्ठो भूतपूर्वः = गौष्ठीनो देशः 'a place which was formerly a Goshtha'. Otherwise when not meaning formerly, we have गोष्ठो वर्तते ॥,