What is the sexual orientation of Nick Carraway?

Possibly. There is textual evidence that supports this claim. For example, on page 36, Nick sees Mr. McKee "asleep on a chair with his fists clenched in his lap, like a photograph of a man of action. Taking out my handkerchief I wiped from his cheek the remains of the spot of dried lather that had worried me all the afternoon".

On the next page lies the elevator scene with Nick and Mr.McKee. What you need to know is the "lever" means a male genital part. It goes:

"Come to lunch some day," he suggested, as we groaned down in the elevator.

"Where?"

"Anywhere."

"Keep your hands off the lever," snapped the elevator boy.

"I beg your pardon," said Mr. McKee with dignity, "I didn't know I was touching it."

"All right," I agreed, "I'll be glad to."

... I was standing beside his bed and he was sitting up between the sheets, clad in his underwear, with a great portfolio in his hands.

"Beauty and the Beast... Lonliness... Old Grocery Horse... Brook'n Bridge..."

Then I was lying half asleep in the cold lower level of the Pennsylvania Station, staring at the morning Tribune, and waiting for the four o'clock train.

There is more, but that is basically the most obvious part.