why would Daisy be so indiscreet with Gatsby that Tom finds out about them?

Daisy is overwhelmed by Gatsby's attention. Gatsby symbolizes the past, something she wanted that she didn't get (at least then), and in the present....... she has everything she could possibly want except love. For Daisy, Gatsby was a return to the emotions of young love. He made her feel as if she were more than just the wife and mother she represented to her husband.

As for her indicretion...... I believe she wanted Tom to find out. Leaving her husband was not something I believe she would ever have seriously considered, but she did want to wake him up.