In plain words
Such a person has nothing to gain from action here and nothing to lose by inaction, and depends on no being for any purpose.
Word for word
For him there is no gain at all by what is done, nor any [loss] here by what is not done; nor has he, among all beings, any dependence for a purpose.
The commentators
Bhagavadgītābhāṣyam
किञ्च —
नैव तस्य परमात्मरतेः कृतेन कर्मणा अर्थः प्रयोजनमस्ति । अस्तु तर्हि अकृतेन अकरणेन प्रत्यवायाख्यः अनर्थः, न अकृतेन इह लोके कश्चन कश्चिदपि प्रत्यवायप्राप्तिरूपः आत्महानिलक्षणो वा नैव अस्ति । न च अस्य सर्वभूतेषु ब्रह्मादिस्थावरान्तेषु भूतेषु कश्चित् अर्थव्यपाश्रयः प्रयोजननिमित्तक्रियासाध्यः व्यपाश्रयः व्यपाश्रयणम् आलम्बनं कञ्चित् भूतविशेषमाश्रित्य न साध्यः कश्चिदर्थः अस्ति, येन तदर्था क्रिया अनुष्ठेया स्यात् । न त्वम् एतस्मिन् सर्वतःसम्प्लुतोदकस्थानीये सम्यग्दर्शने वर्तसे ॥ १८ ॥
In plain words
And there is more —
For someone who has found his delight in the supreme Self, action has absolutely no purpose left to serve. 'Well then,' you might object, 'surely not acting brings its own fault, a kind of failure.' No — not acting brings him no fault of any kind, here in this world, not the smallest transgression, not any loss to himself, none at all. And there is nothing more to it than that: among every being there is, from Brahmā down to the plants rooted in the earth, there is not one thing he still depends on, not one purpose action could still serve him, not one object he leans on the way action leans on a reason. You, Arjuna, are not standing where he stands — in that state like a reservoir flooded full on every side, where nothing more is needed. You are not yet living in that vision.
Close to the Sanskrit
And what more —
Not at all does there exist, for him who delights in the supreme Self, any purpose in what is done, in action. If it be said, then, that by what is not done, by non-performance, there is the fault called transgression — no, by not-doing, here in this world, no fault of any kind whatsoever, whether in the form of incurring transgression or characterized by loss to himself, exists for him at all. Nor, again, is there among all beings — beginning with Brahmā and ending with the immovable — among beings, any object of dependence, attainable by action undertaken for a purpose — any resort, any dependence, any support — resting on some particular being, that is to be accomplished, by which an action having that as its purpose would need to be undertaken. You do not yet stand in this state, likened to a reservoir flooded with water on every side — in right vision.