We'll have to try to figure out what is different about your new grapefruit tree or its situation that is causing the extreme leaf drop. Of course your new tree is not yet established. Some transplant shock is to be expected. Also, dropping of some older leaves when the tree is blooming in spring is normal, but the problem seems more serious than that. You are right to be so concerned about the leaf drop.
Plants feed themselves through the process called photosynthesis.
Grapefruit trees do this using their leaves. If your tree has no leaves, it will be unable to feed itself. Citrus are very particular about that. Their roots will not tolerate standing water, nor will they tolerate drying out. Too much or too little water is one likely cause of serious leaf drop.