What does Gatsby offer Nick in return for his cooperation in inviting Daisy to his (Nick's) house.
Jay Gatsby's actions in this regard indicate how absolutely overwhelmed he is by the idea of meeting Daisy - it would be a dream come true. It is clear, however, that he does not want to make too much of a fuss about the meeting, as he told Jordan Baker:
‘I don’t want to do anything out of the way!’ he kept saying. ‘I want to see her right next door.’
'Right next door' is obviously Nick's place. When Nick and Jay meet later to discuss the arrangement, Jay feigns a careless attitude, but it is clear that he wishes to please Nick. He promises:
“I don’t want to put you to any trouble, you see.”
The benefit to Nick would be that Gatsby would have his grass cut, obviously to create a favourable impression.
“I want to get the grass cut,” he said.
There would, however, be some greater benefit as well, as Jay so hesitantly points out:
“Well, this would interest you. It wouldn’t take up much of your time and you might pick up a nice bit of money. It happens to be a rather confidential sort of thing.”
He says this after confirming that Nick does not make much money and that he is in the 'bond-business'. This is clearly an invitation to Nick to render some service in the industry (probably to sell junk bonds) and to make a fair amount of money. The fact that it is 'confidential' suggests something not entirely above-board. Nick, however, cuts him short by saying:
“I’ve got my hands full,” ... I’m much obliged but I couldn’t take on any more work.”
Even Jay's assurance that Nick won't have to work with Meyer Wolfsheim, does not sway him and Jay, after waiting a while for some type of response, which Nick does not offer, reluctantly leaves.