Bhashya
सर्वं यथोक्तं गार्ग्या प्रत्युच्चार्य तमेव पूर्वोक्तमर्थमवधारि-तवान् आकाश एवेति याज्ञवल्क्यः ।गार्ग्याह — कस्मिन्नुखल्वाकाश ओतश्च प्रोतश्चेति ।आकाशमेव तावत्कालत्र-यातीतत्वात् दुर्वाच्यम्, ततोऽपि कष्टतरम् अक्षरम्, य-स्मिन्नाकाशमोतं च प्रोतं च, अतः अवाच्यम्- इति कृत्वा,न प्रतिपद्यते सा अप्रतिपत्तिर्नाम निग्रहस्थानं तार्किकस-मये ; अथ अवाच्यमपि वक्ष्यति, तथापि विप्रतिपत्तिर्नामनिग्रहस्थानम् ; विरुद्धा प्रतिपत्तिर्हि सा, यदवाच्यस्य वद-नम् ; अतो दुर्बचनम् प्रश्नं मन्यते गार्गी ॥
In plain words
Yājñavalkya repeated her whole question back to her and confirmed his answer: space, and space alone. 'Then what,' Gārgī pressed, 'is space itself woven into?' Here is the trap she saw: space is already beyond time, so it resists being put into words. And whatever space is woven into — the Imperishable — is harder still to describe. Call it unspeakable and stop there, and by the logicians' own rules, that silence counts as losing the argument. But try to speak the unspeakable anyway, and that counts as losing too, because saying the unsayable contradicts itself. Either way, she loses. That is exactly why Gārgī calls this a hard question.
Close to the Sanskrit
Having repeated everything just as stated by Gārgī, Yājñavalkya confirmed that very previously-stated meaning: 'Ether, indeed.' Gārgī said: 'But in what, then, is ether woven, crosswise and lengthwise?' Ether itself, first of all, being beyond the three times, is hard to state; harder still than that is the Imperishable, in which ether is woven, crosswise and lengthwise; therefore it is unspeakable — holding this view, she would give no answer; that non-answering is, in the logicians' convention, called a point of defeat. But if he goes on to state even the unspeakable, that too is a point of defeat, called misstatement — for that is a contradictory assertion, namely the speaking of what cannot be spoken. Therefore Gārgī considers her question a hard one to answer.