Optimis facinoribus variis in conspectu omnium praestandis rex apud populum famam pietatis ac sapientiae possedit.
Is praestandis a gerund or gerundive or both? And does it matter for the translation?
The gerund is only used in the singular, so for that reason alone praestandis has to be a gerundive.
The gerund is a verbal noun (and as a subject or object is replaced by the infinitive). The gerundive is a verbal adjective; it adapts its form to the gender, number, and case of whatever it modifies. There is no good analogue for this distinction in English, so typically a fluent translation to English shows no signs of whether a gerund or a gerundive was used. But to get the translation right it is important to understand the function of the gerund or gerundive in the sentence, and therefore the distinction is important to maintain.