What is the irony of Mr. Gatz’s admiration of the house?
I think the irony of Mr. Gatz' admiration for the house, was that to Gatsby, the material possessions meant nothing, his success meant nothing without Daisy. Gatsby bought the house, the clothes, and all of his material goods to be closer to Daisy and her crowd, but the actually house meant nothing to him, other than as a tool to lure Daisy. His father, on the other hand, sees the house as Gatsby's accomplishment, when really Gatsby died in pursuit of the only accomplishment he only wanted: Daisy.
Like others whom Nick grow to dislike, Mr. Gatz sees the material good as what Gatsby accomplished with his life, rather than for his personality or drive. Even the lower classes, Fitzgerald is saying, cannot help but be pulled in by the lure of material goods.
Gatsby's dad views the house as an indication of his son's success. Mr. Gatz has Jimmy's old daily schedule from Gatsby's childhood and talks about how he always knew Jimmy would make something of himself. What Mr. Gatz doesn't realize, is that it was all of this money that eventually led to his death. The big house and lavish parties were Gatsby's way of reuniting with Daisy, an act that eventually led to Wilson shooting him.
When Mr. Gatz is admiring and praising this house as an icon of James's success, he should be lamenting and cursing this house and all it stood for for bringing about the demise of his son.