Is the word "being" used here as a gerund? If so, can I use infinitive (to be) here?
She has a conception of people as being basically good.
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The function of "being basically good" in this sentence is to describe the conception of people that "she" has, so you can think of it as a kind of adjective. So the "-ing" or "being" isn't meant to convey a tense, but rather to make an adjective out of the verb. The infinitive, at least to my knowledge, can't be used in an adjective context, so "as to be basically good" would be ungrammatical in your example.
You can also use "-ing" to make a noun out of the verb, as in this example:
Being a good person takes a lot of patience.
In this sentence, "being a good person" is the subject and functions as a noun phrase. Note that you can also have a noun phrase using the infinitive:
To be a good person is to have a lot of patience.
"To be a good person" here is the subject of the sentence, and "to have a lot of patience" is the object.