In Chapter Four of The Great Gatsby, Nick Carraway's admiration of the quest for the ideal and romantic reaction to Jay Gatsby's purchase of the house in West Egg for the sole purpose of being near Daisy Buchanan foreshadows the ending lines of the novel:
Gatsby believed in the green light, orgastic future that year by year recedes before us. It eluded us then, but that's no matter...
Despite his knowing that Jay Gatsby's personal history is fabricated, Gatsby emerges for Nick Carraway as the Romantic Ideal. He pursues his "holy grail" and attains mythological proportions as his car is likened to the flight of Icarus soaring through the Valley of Ashes "with fenders spread like wings."
This statement of Nick about Jay Gatsby in Chapter Four underscores the themes of aspiration and the theme of the ideal. For him, Gatsby's car's "three-noted horn" trumpets the approach of royalty among the decadent, materialistic crowd that frequents Gatsby's parties.
Nick Carraway says this about Gatsby. He says it when he finds out from Jordan Baker that Gatsby has bought the mansion just so that he could be across the bay from Daisy. What the quote that you mention means is that this made a big difference in how Nick viewed Gatsby.
Up until this point in the book, it had not been clear to Nick why Gatsby acted the way he did. He had no idea why Gatsby wanted the life that he had. But now, talking to Jordan, he understood that Gatsby was trying to get Daisy. That seemed like a much more understandable and a much more human motive for having this big house and the parties and such.
So now Gatsby came alive for Nick. Gatsby's purpose in life became much clearer and Nick understood him and liked him better than he had before.
In chapter 1, Nick (speaker of the line you mention) describes Gatsby from the perspective of time--after that long summer had passed. One of the first things he says about Gatsby is that "he represented everything for which I have an unaffected scorn." But then, just a few lines later, Nick says this:
"...there was something gorgeous about him, some heightened sensitivity to the promises of life...--it was an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again."
For me, your quote is the moment of revelation when Nick understands that Gatsby is not just a newly rich, socially inept businessman who wastes his time and money on things that don't matter--the things for which Nick has no respect. When Nick says this line after hearing the history between Jay and Daisy, Gatsby has become a fellow man chasing the dream of a girl he once--and still--loved. It's this moment which bridges the gap between "unaffected scorn" and "romantic readiness."