grammar - How do you use adjective or adverb to modify the gerund?

You need to fix your sentences.

Consider, for your first alternative:

1. Regular exercising enhances one's self-confidence.

The "exercising" ing-form here is closer to a noun in meaning, thus you should use the adjective "regular." BTW, I refuse to use anymore the old classification in gerunds and participles (as does that grammar bible of Quirk et al, 1985), as I find it counterproductive.

Syntactically, "Regular exercising" is a noun phrase acting as a subject.

Now, for your other alternative:

2. Enhance your self-confidence by exercising regularly.




3. By exercising regularly, you can enhance your self-confidence.

The "exercising" ing-form in 2 and 3 is closer to a verb/action in meaning, thus you should use the adverb "regularly."

BTW, in 2 "enhance" is an imperative.

Also, syntactically "by exercising regularly" is an adverbial clause.