"I'll reply about this later.", is this correct grammatically and right usage of the proposition "about"?
or should I have used "to"?
You reply to people and you respond to their messages or queries.
You can reply to them about a particular issue or problem or question.
More often, you would say: Concerning your question on.......
Yes, you could say: I'll reply about this later.
A more informal way would be to say: I'll get/come back to you about this later.