When Nick makes this comment in Chapter Eight, he is saying that Gatsby has "wooed" Daisy "under false pretenses" since he does not come from the same social background as her. He does not come from old money, for example, nor does he have a strong family background. Remember that Jay Gatsby is nothing more than an invented persona and this his wealth and success are derived from immoral and questionable sources.
But Gatsby does not share this view. When it comes to his relationship with Daisy, Gatsby is trying to "recapture the past." He is trying to pick up where they left off in Louisville. Everything he has done to achieve wealth and success was done for Daisy. In Gatsby's mind, Daisy is like the Holy Grail. She is the reason for his existence, and he cannot let go. His love for her is genuine and everlasting, so the idea of false pretenses does not enter into Gatsby's mind.
From Gatsby's perspective, then, he has not wooed Daisy under any false pretenses. In contrast, he has created the wealth to provide everything she will ever need and already feels as though they are married.
Gatsby doesn't feel he is operating "under false pretenses." In his steadfast blindness to everything that does not support and contribute to his perception of the ideal American dream he has created for himself, he is certain that Daisy will eventually come to love him in the same way that he loves her.
As a result of this determination and dedication to the achievement of this goal, Gatsby creates situations in which he and Daisy can come together and fantasizes about other circumstances in which their paths crossed, but not in the way he chooses to remember. To him, however, it is reality, and nothing anyone else says or does can take it away from him. There is no falsehood involved, as Gatsby sees it.
I gathered that he wanted to recover something, some idea of himself perhaps, that had gone into loving Daisy. His life had been confused and disordered since then, but if he could once return to a certain starting place and go over it all slowly, he could find out what that thing was...