In plain words
That unmanifest is called the Imperishable; they call it the supreme goal. Having reached it, none return. That is My supreme abode.
Word for word
The unmanifest, said [to be] the Imperishable, they call it the supreme goal; having reached which they do not return, that [is] My supreme abode.
अव्यक्तोऽक्षर इत्युक्तस्तमाहुः परमां गतिम्यं प्राप्य न निवर्तन्ते तद् धाम परमं मम
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
योऽसौ अव्यक्तः अक्षरः इत्युक्तः, तमेव अक्षरसंज्ञकम् अव्यक्तं भावम् आहुः परमां प्रकृष्टां गतिम् । यं परं भावं प्राप्य गत्वा न निवर्तन्ते संसाराय, तत् धाम स्थानं परमं प्रकृष्टं मम, विष्णोः परमं पदमित्यर्थः ॥ २१ ॥
In plain words
The one already spoken of as the unmanifest, the imperishable — that same unmanifest reality, the one that carries the name akṣara, is what the knowers call the highest goal, the destination past which there is nothing further to reach. Go there, arrive at that supreme state, and you do not come back — saṃsāra has no return road from it. That abode, that place, highest and unsurpassed, is Mine: the supreme station of Viṣṇu. That is what the verse means.
Close to the Sanskrit
That which was called 'the unmanifest, the imperishable' — that very unmanifest state, bearing the name 'imperishable', they call the highest, the most excellent, goal. Having reached — having gone to — which supreme state they do not return for saṃsāra: that abode, that place, supreme, most excellent, is Mine — the supreme station of Viṣṇu is the meaning.