In plain words
Among all trees I am the ashvattha; among divine seers I am Narada; among gandharvas I am Chitraratha; among perfected beings I am the sage Kapila.
Word for word
[I am] the aśvattha among all trees, and Nārada among the divine seers; among the gandharvas Citraratha; among the perfected, the sage Kapila.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
अश्वत्थः सर्ववृक्षाणाम् , देवर्षीणां च नारदः देवाः एव सन्तः ऋषित्वं प्राप्ताः मन्त्रदर्शित्वात्ते देवर्षयः, तेषां नारदः अस्मि । गन्धर्वाणां चित्ररथः नाम गन्धर्वः अस्मि । सिद्धानां जन्मनैव धर्मज्ञानवैराग्यैश्वर्यातिशयं प्राप्तानां कपिलो मुनिः ॥ २६ ॥
In plain words
Among all trees, the aśvattha; and among the deva-ṛṣis, Nārada. These are gods already, and they became seers on top of that, because the mantras are shown to them directly — that is what the word deva-ṛṣi means. Of them, I am Nārada. Among the gandharvas, I am the one called Citraratha. Among the siddhas — those who came into the world already holding dharma, knowledge, dispassion, and mastery at their highest pitch, with nothing left to acquire — I am Kapila the sage.
Close to the Sanskrit
The aśvattha among all trees, and among the deva-ṛṣis, Nārada. Being gods indeed, they have attained the state of ṛṣi because they are seers of mantras; those are the deva-ṛṣis. Among them, I am Nārada. Among the gandharvas I am the gandharva named Citraratha. Among the siddhas — of those who by birth alone have attained a surpassing excellence of dharma, knowledge, dispassion, and lordship — I am Kapila the sage.