gewiss
certain
Means sure, unquestionable, or established. It expresses a high degree of certainty.
Nach dieser Leistung ist ihm der Sieg so gut wie gewiss.
After that performance, victory is as good as certain for him.

certain
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certain
Means sure, unquestionable, or established. It expresses a high degree of certainty.
Nach dieser Leistung ist ihm der Sieg so gut wie gewiss.
After that performance, victory is as good as certain for him.

'Gewiss' (certain) is closely related to the verb 'wissen' (to know). The basic idea is that something is 'gewusst' (known) and therefore certain. The word family also includes 'die Gewissheit' (certainty) and 'sich vergewissern' (to make sure).
Ich muss mich erst vergewissern, ob die Information stimmt.
I first have to make sure whether the information is correct.
The meaning of 'gewiss' (certain) changes significantly depending on its position in the sentence. If it comes after a verb (predicative), it means 'sicher' (certain/sure). If it comes before a noun (attributive), it means 'ein bestimmter' (a certain/particular).
Der Erfolg ist ihm gewiss.
Success is certain for him.
In the meaning 'sicher' (certain/sure), 'gewiss' is often used with the verb 'sein' (to be). The person for whom something is certain is in the dative.
Mit deiner Hilfe ist uns die Unterstützung gewiss.
With your help, support is certain for us.
When 'gewiss' (certain) comes before a noun, it expresses that a person or thing is not known in detail or is not meant to be named. It is declined like a normal adjective.
Er hat ab einem gewissen Alter graue Haare bekommen.
He started getting gray hair after a certain age.
As an adverb, 'gewiss' (certainly) means something like 'sicherlich' (certainly) or 'bestimmt' (definitely) and strengthens a statement. It often appears at the beginning or in the middle of a sentence.
Das wird gewiss nicht einfach, aber wir schaffen das.
That certainly won't be easy, but we can do it.
natural / of course
'natürlich' and 'gewiss' can both confirm a statement and then mean roughly 'certainly' or 'of course'. 'natürlich' sounds very normal in everyday language and often friendly, especially as an answer to a request or question. As an adverb, 'gewiss' sounds more elevated or somewhat old-fashioned. It is used more when you want to confirm a statement in a more solemn or stronger way. In conversation, 'natürlich' is usually the more natural choice.

Sie wird gewiss bald antworten, wenn sie die Nachricht liest.
She will certainly reply soon when she reads the message.
Natürlich helfe ich dir heute Abend beim Lernen.
Of course, I’ll help you study this evening.