What was Gatsby's original reason for dating Daisy? How and why did his plan change once he got to know her?

The reason behind Gatsby dating Daisy was mainly because of economic purpose.

Explanation:

There are some items to be taken into account; we see that Gatsby is presented as a character who is just focused on whatever is related to money and wealth. Therefore, when he set his eyes on Daisy, it was only because of her wealth, but he made it clear that he was not in her level, but managed to make her believe that he could actually take care of her.