In plain words
Among nagas I am Ananta; among beings of the waters I am Varuna; among the ancestors I am Aryaman; among those who restrain I am Yama.
Word for word
And I am Ananta among the nāgas; Varuṇa among the beings of the waters [am] I; among the ancestors I am Aryaman, and Yama among those who restrain [am] I.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
अनन्तश्च अस्मि नागानां नागविशेषाणां नागराजश्च अस्मि । वरुणो यादसाम् अहम् अब्देवतानां राजा अहम् । पितॄणाम् अर्यमा नाम पितृराजश्च अस्मि । यमः संयमतां संयमनं कुर्वताम् अहम् ॥ २९ ॥
In plain words
And among the nāgas, I am Ananta, their very king. Among aquatic beings, I am Varuṇa, king of the deities of the waters. Among the ancestors, I am the one called Aryaman, their king. And among all those who exercise restraint and control, I am Yama.
Close to the Sanskrit
And I am Ananta, of nāgas — of the particular kinds of nāgas — and I am the king of the nāgas. Varuṇa, of aquatic beings, I, of the deities of water, the king, I. Of the ancestors, named Aryaman, I am the king of the ancestors. Yama, of those who restrain — of those exercising restraint — I.