There is some confusion about what all these words actually mean, so I am posting this to clarify things.
”Open Pollenated”, or OP, means that a plant will come “true” from seed; the next generation will look reasonably like the last, so seed can be saved from them. Wild plants are generally this way, as are most traditional plants.
“Heirloom” means that the OP strain in question is about sixty years old. Every heirloom plant was new once.
“Hybrid” means that two OPs were crossed to grow the seed. If a zucchini and a pumpkin were planted next to one another, and the seed was saved, it would almost certainly be hybrid seed; bees would have crossed it. The seed you planted next year would all come up looking alike. If it was better than the originals in some way, the gardener might decide to do it again on purpose. If, however, seeds were saved from the plants grown from the hybrid seed, every resulting plant would be wildly different. Selection over may years could then create a new OP variety which would come true from seed. Most heirlooms were once hybrids. So there is nothing wrong with a hybrid. The problem comes from their modern use. Since nobody else can duplicate a hybrid, companies dropped all their traditional heirloom varieties and switched over to them, thus greatly diminishing vegetable diversity, making famers dependent on bought seed, and setting the stage for disasters similar to the Irish Potato Famine ( at that time there were only two potato varieties in Ireland.) Also, modern varieties of any sort, OPs or hybrids, are suited to modern agriculture, and need large amounts of chemical fertilizer, pesticides, and perfect growing conditions. Heirlooms are better suited to low input methods.
You might be interested to know that apple seeds do not come true, since almost all of them are hybrids. You need two different trees for pollination to occur successfully.
GMO are a whole different matter. They are created by scientists in a Lab, and are dangerous on many different levels. Currently they are used to increase the control of large corporations over the food supply. However, unlike hybrids, which can be used rightly, GMOs have their own inherent risks.
I think that Heirloom varieties should be saved, because they represent the wisdom of the past; that small farmers should breed their own new OPs and hybrids, to adapt to changing conditions and methods, and to bring unique products to market; and that GMOs should be opposed in all situations.