Gatsby disappears when which other character unexpectedly arrives at lunch? Why Gatsby doesn't want to see this character?

Gatsby quickly disappears when Nick bumps into Tom in the restaurant. He doesn't want to see Tom because Tom is Daisy's husband.

When he saw us Tom jumped up and took half a dozen steps in our direction.

“Where’ve you been?” he demanded eagerly. “Daisy’s furious because you haven’t called up.”

“This is Mr. Gatsby, Mr. Buchanan.”

They shook hands briefly, and a strained, unfamiliar look of embarrassment came over Gatsby’s face.

“How’ve you been, anyhow?” demanded Tom of me. “How’d you happen to come up this far to eat?”

“I’ve been having lunch with Mr. Gatsby.”

I turned toward Mr. Gatsby, but he was no longer there.