그렇군요 is a better expression for this phrase.
아.. 그렇군요 = *Oh.. (It is) so.
아.. 그렇구나 (for those younger or same age as you).
"I see" as in I understand would be 알겠어(요) or 알았어(요) [informal polite]
also 알겠습니다 [formal]
Informally you can just say "응"(but only to very close friends)