Talk or talking

The sentence you came across " All you have to do is talk back " is absolutely grammatically correct as it stands for the sentence " All you have to do is to talk back. "
However, the sentnce remains equally grammatically correct if " talking back " is used in lieu, because you have just used a noun phrase that makes the sentence sound clumsy, though the meaning thereof remains
unchanged.

You're correct to say "I regard [talk back] as a whole thing."
or
You're correct in saying "I regard [talk back] as a whole thing."

It is a phrasal verb.

talk back = 駁嘴