If you want to know about the appearance of Gatsby's house, there is some description of it at the beginning of Chapter 5 telling the reader that the house sits at the end of the peninsula and has many windows. Then about a third of the way into the chapter, while Nick is giving Jay and Daisy some time together and he looks across the lawn to Gatsby's house, the reader learns the style of the house is "period" (in this case, the house probably resembled a large manor home such as one might find in England) and it was built by a brewer. Also the reader learns there is a large centrally placed bay window. In the last six or so pages of the chapter, when Jay is showing Daisy through his house, the reader learns that there are gardens around the house and marble steps leading up to the main entrance. In the house, the reader is told that the interior design varies somewhat from room to room with "Marie Antoinette music rooms and Restoration salons". There is a large library and upstairs the bedrooms are decorated in period styles. Gatsby's suite of rooms include a bedroom, a bath, and an "Adam study". Gatsby's bedroom, though simple in decor, has two large cabinets that he opens, then he pulls his shirts from them.
It was built on a peninsula
Gatsby says to Nick in chapter 5 "It took me three years to earn the money to buy the house".