Jay Gatsby spoils Nick Carraway, a lot. Always takes him out for really expensive dates and buys him stuff that is extremely pricey.
And Nick is totally flattered by it, but at the same time he feels bad because he doesn’t want Jay spending all of this on him and besides, he can’t pay him back. Nick is by no means poor, but he definitely has less fortune than his boyfriend, and he certainly can’t afford the same stuff that Gatsby can.
Sure, Gatsby can afford everything just fine without having to be paid back but that fact doesn’t really ease Nick’s guilt.
So one day after Gatsby does or gets something extremely extravagant and costly for Nick, Nick just freaks out like, “I love you and I love this but at the same time just please actually buy me something that’s cheap because I feel so so awful about being unable to ever pay you back for this stuff!” And then Gatsby just smiles and asks Nick, “Doesn’t this make you happy, though?” and Nick is all like “yes, but-” and then Gatsby says, “in that case, old sport, you’ve already repaid me, because your happiness is the best gift I could ask for.”
(Nick then rolls his eyes and calls Gatsby the most corny man he’s ever met, but he also doesn’t let prices bother him too much after that.)