Simple subject

Hi: I'd like to ask you a few things about what is called the "Simple Subject" in English grammar, please. I've learnt that a "simple subject" is a Noun or a Pronoun without any modifers or descriptive words that functions as the subject of a sentence; that performs or does the action expressed by an action verb. Or it the Noun or the Pronoun without any modifiers that is connected or linked to a "subjective verb complement"
when the verb it precedes is a linking verb.
But I'd like to know if we can also refer to a "gerund" or an "infinitive", or a "Noun Clause", for example, when these things are used as
the subject of a sentence as a simple subject.

Here are some examples:

- Walking is good for your health. (Here, the gerund "walking" is used as the subject of the sentence. Can this be referred to as
a simple subject, since it has no modifiers accompanying it?)

-To err is human, to forgive is divine. (Here, the infinitives "to err" and "to forgive" are used as the subjects, but they are not accompanied
by any modifiers. For this reason, can we call them "simple subjects"?)

- What he does doesn't impress me at all. (Here, the "noun clause" "what he does" is the subject of the sentence, but it not accompanied by
any modifiers. So, is it a simple subject, here?)
Thank you.