When Gatsby first met Daisy before the war, he was not in love with her. Having come from such humble beginnings himself, Gatsby
was overwhelmingly aware of the youth and mystery that wealth imprisons and preserves, of the freshness of many clothes and of Daisy, gleaming like silver, safe and proud above the hot struggles of the poor.
To him, Daisy represented everything that he wanted: wealth, status, ease, safety, and financial security. Her house was luxurious, and her porch was bright with starlight. Even the furniture squeaks "fashionably" when she shifts her body on top of it. It is these things that Gatsby wants at first: this brightness, this certainty and confidence, and this fashionability.
However, as he tells Nick, "I can't describe to you how surprised I was to find out that I loved her." He did not intend to fall in love with Daisy, but he did anyway. He hoped that she would break it off with him, making his own life easier, but she did not because she returned his feelings. He knew that he was deceiving her, that she believed he came from the same background as she did—his own origins obscured by his military uniform and manners—but he simply could not bring himself to break the spell between them.
So, in the beginning of their relationship, it was not serious for Gatsby, but it quickly became serious when he found that he had fallen in love with her and she with him.