Seedless watermelon and grapes aren't GMOs.

What fruits are genetically modified?

A few fresh fruits and vegetables are available in GMO varieties, including potatoes, summer squash, apples, and papayas. Although GMOs are in a lot of the foods we eat, most of the GMO crops grown in the United States are used for animal food.

How have grapes been genetically modified?

The process of growing seedless grapes uses a form of asexual reproduction. The new plant is essentially a genetic clone of the original parent plant which requires manual manipulation with the plants genetic material.