The Great Gatsby, "But if I do"

I'd like to know if the sentence "But if I do" in the following excerpt from the last chapter of Gatsby is complete or no and means "But in this case I will do it" or just "But in this case", "'if I see anybody"?

"The funeral's tomorrow," I said. "Three o'clock, here at the house. I wish you'd tell anybody who'd be interested."

"Oh, I will," he broke out hastily. "Of course I'm not likely to see anybody, but if I do."

Thank you.