In plain words
This threefold tapas, practiced with supreme faith by people of collected mind who seek no fruit, is called sāttvika.
Word for word
That threefold tapas, undergone with supreme śraddhā by people not craving fruit [and] collected, they call sāttvika.
श्रद्धया परया तप्तं तपस्तत् त्रिविधं नरैःअफलाकाङ्क्षिभिर्युक्तैः सात्त्विकं परिचक्षते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
यथोक्तं कायिकं वाचिकं मानसं च तपः तप्तं नरैः सत्त्वादिगुणभेदेन कथं त्रिविधं भवतीति, उच्यते —
श्रद्धया आस्तिक्यबुद्ध्या परया प्रकृष्टया तप्तम् अनुष्ठितं तपः तत् प्रकृतं त्रिविधं त्रिप्रकारं त्र्यधिष्ठानं नरैः अनुष्ठातृभिः अफलाकाङ्क्षिभिः फलाकाङ्क्षारहितैः युक्तैः समाहितैः — यत् ईदृशं तपः, तत् सात्त्विकं सत्त्वनिर्वृत्तं परिचक्षते कथयन्ति शिष्टाः ॥ १७ ॥
In plain words
The austerity of body, speech, and mind has just been described. Now: when people actually practise it, how does it fall into three kinds according to which guṇa is at work — sattva and the rest? That is what is said next:
Practiced with faith -- that is, with a real conviction that it matters -- with the highest, most excellent faith -- carried out, performed -- that austerity, the very one already described as threefold, resting on the three grounds of body, speech, and mind -- by people, by practitioners, who want nothing back from it, entirely free of craving any result, disciplined, and deeply focused -- whatever austerity looks like that, the wise call it sattvic, born of sattva.
Close to the Sanskrit
As to how the bodily, verbal, and mental austerity described above, when practised by men, becomes threefold through the difference of the guṇas beginning with sattva — it is said:
With faith, i.e. with the conviction of existence, with supreme, i.e. surpassing, [faith] -- practiced, performed -- that austerity, the one under discussion, threefold, of three kinds, having three bases -- by men, by practitioners, who do not desire the fruit, [i.e.] free from craving for the result, [who are] disciplined, focused -- whatever austerity is of this kind, that they, the learned, designate as sattvic, produced from sattva.