“Of course, of course! They’re fine!” and he added hollowly, “. . .old sport.”
Gatsby’s excitement, bordering panic, can be seen in these exclamations. He is feverish over meeting Daisy and is losing self-control.
This is immediately contradicted by the adverb ‘hollowly’, which suggests that for all his excitement, he actually lacks substance. This is reinforced by the ghostly ellipsis preceeding ‘old sport’, which turns the phrase into something threadbare and lacking vitality. In many ways, Gatsby is (SPOILER ALERT!!!) already dead.
To help improve the meaning of these lyrics, visit “The Great Gatsby (Chapter V)” by F. Scott Fitzgerald and leave a comment on the lyrics box