In plain words
Among purifiers I am the wind; among wielders of weapons I am Rama; among aquatic creatures I am the makara; among rivers I am the Ganga.
Word for word
The wind among purifiers I am; Rāma among wielders of weapons [am] I; among creatures of the water I am the makara, and among streams I am Jāhnavī.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
पवनः वायुः पवतां पावयितॄणाम् अस्मि । रामः शस्त्रभृताम् अहं शस्त्राणां धारयितॄणां दाशरथिः रामः अहम् । झषाणां मत्स्यादीनां मकरः नाम जातिविशेषः अहम् । स्रोतसां स्रवन्तीनाम् अस्मि जाह्नवी गङ्गा ॥ ३१ ॥
In plain words
Among things that blow and purify, I am Pavana, the wind. Among weapon-bearers, I am Rāma — Rāma the son of Daśaratha. Among fish and every such creature, I am the one called Makara, a particular species of sea-creature. And among flowing rivers, I am Jāhnavī, the Ganges herself.
Close to the Sanskrit
Wind, air, of those that blow, of purifiers, I am. Rāma, of weapon-bearers, I — of weapons, of holders — the son of Daśaratha, Rāma, I. Of fish, beginning with fish, named Makara, a particular species, I. Of streams, of flowing rivers, I am Jāhnavī, the Ganges.