The affix yat comes in the sense of 'who obtains it' after the words 1. dhana and 2. gaṇa being in the Accusative -2nd case in construction.,
Thus धनं लब्धा = धन्यः, so also गण्यः ॥ लब्धृ is formed from लभ् by तृन्, hence it takes the accusative.,
धन-गणं लब्धा
Adhyāya 4 · Pāda 4 · Rule 84