In chapter 9 of The Great Gatsby, what is the irony of Mr. Gatz's comment about James Hill?

At the funeral of Gatsby, not many people come. However,
Gatsby's father comes, who obviously is proud of his son. He has a conversation with Nick. As
they talk, Mr. Gatz says that Gatsby had a great future ahead of him and that had he lived
longer, he would have helped America, like James J. Hill.

Hill was
the CEO of a family of lines headed by the Great Northern Railway. He built trains that brought
the country together.

The irony is that Mr. Gatz had very little
true knowledge of his son. There was little philanthropy in him. Mr. Gatz
says:

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‘He had a
big future before him, you know. He was only a
young man but he had a lot of brain
power here.’
He touched his head impressively and I nodded.
‘If he’d of lived
he’d of been a great man. A man like
James J. Hill. He’d of helped build up the
country.’
‘That’s true,’ I said,
uncomfortably.

Gatsby did all he did
to impress Daisy. This is why Nick uncomfortably agreed. He knew Gatsby better than Gatsby's
father.