How can seedless fruits not be GMO?
GMO stands for Genetically Modified Organism. 

Grafting a plant to a rootstock of another plant is a form of plant husbandry -- a farming technique -- but this isn't changing the DNA code of either of the plants. Sort of like you receiving a donated organ wouldn't be the same as Genetically Modifying you. When a desired trait arises in a plant this is a way to keep reproducing it without having to wait for cuttings or seeds to grow. 

There are some plants such as seedless watermelon that are reliably made by crossing two seed species. You take two species of watermelon, Species 1 and Species 2. Both of these are normal -- they make seeds. When you cross them, you get seeds that grow into AB hybrids, seedless watermelons. These AB are infertile, so you have to cross A and B again each year to get more seeds to plant. 

Species 2 has twice the number of chromosomes as 1. They are able to cross and make seeds that grow. But when the [1x2] plants grow and try to form seeds, those cells have an odd number of chromosomes. The process of making seeds and pollen -- meiosis -- has a step when half of the chromosomes go into one cell and half in another. Because there is an odd number, the process never finishes and you get no seeds. 

to demonstrate how this doesn't work, lets look as a simple example. 
Species 1 has two chromosomes [A and B]. It has two copies of each chromosome in all of its cells (one from each parent), and just one copy in its sex cells

Species 1 Normal Cells [AA BB] and sex cells [A B]

Now, Species 2 has a different set up. It has twice the chromosome number
Its normal cells are [aaaa bbbb] and its sex cells are [aa bb]

this means that when you cross pollen from one with seed from the other, you get 
[aaA bbB] or [AAa BBb]

When these crosses try to make seeds, its impossible to halve the triplet chromosomes. 

True GM technology is used to do things such as, in olives taking the genes that cause oil production to occur, and copying them many times. This gives you more oil production.

Or, in tomatoes -- when you pick a tomato the cells of the skin start expressing a gene that makes the skin soften and break apart. This facilitates seed dispersal. GM deletes that gene, making the fruit bruise less, reducing need for packaging and quadrupling the shelf life.