In plain words
But those who, finding fault, do not follow this teaching of Mine—know them to be confused in all knowledge, undiscerning, and lost.
Word for word
But those who, finding fault, do not follow this view of Mine, know them, confused in all knowledge, to be lost [and] undiscerning.
The commentators
Bhagavadgītābhāṣyam
ये तु तद्विपरीताः एतत् मम मतम् अभ्यसूयन्तः निन्दन्तः न अनुतिष्ठन्ति नानुवर्तन्ते मे मतम् , सर्वेषु ज्ञानेषु विविधं मूढाः ते । सर्वज्ञानविमूढान् तान् विद्धि जानीहि
नष्टान् नाशं गतान् अचेतसः अविवेकिनः ॥ ३२ ॥
In plain words
But there are others, the very opposite of that surrendered disciple — people who sneer at this teaching of mine, who tear it down instead of living by it, who never actually practice what I've taught, never fall in step with it. Confused, every which way, in everything they think they know — that's what they are. Know them for exactly that, Arjuna: people lost in confusion about everything.
'Ruined' means: they have gone to their destruction. 'Mindless' means: they are without viveka, without the power to discriminate. ||32||
Close to the Sanskrit
But those who, opposite to that, carping at, reviling, this teaching of mine, do not practice, do not conform to, my teaching — deluded in various ways with regard to all knowledge, they [are]. Know, know, them as deluded with regard to all knowledge.
'Ruined' (naṣṭān): gone to destruction. 'Mindless' (acetasaḥ): without discrimination. ||32||