The term “GMOs” refers to genetically modified organisms, where the modification is made on a cellular level in a laboratory. It is not legal to sell GMO gardening seeds to the general consumer in Canada. Only farmers and large commercial agencies can buy GMO seeds for commercial use. While some home gardening seeds are labelled “GMO-free”, this is simply a marketing tactic, as all garden seeds are GMO-free. In horticulture, selective breeding is used, which is different from creating GMO seed. We'll go into selective breeding soon when we talk about heirloom varieties below. As far as concerns about GMO seeds go, just buy whatever seeds catch your interest as all garden seeds are GMO-free.
When it comes to planting heirloom varieties—the choice is yours. Basically, heirloom seeds include any seeds that grow into a plant that is like their parent (called "breeding true") and have been grown by people for a long time with 50 years a common cut off. While we may assume that heirloom seeds have not been genetically modified, all cultivated seed has had some sort of human-influenced breeding. Not even heirloom seeds look like the “original” plant. It might help to think about how different a poodle and a wolf are. Wolves are the “original” ancestors of poodles but they were bred by humans to look and behave differently. Similarly, wild corn looked much like grass, cabbage was smaller than a tiny pea, and watermelon was mostly dense rind.
It’s important to note that while heirloom seeds do tend to have more variety in taste and texture, they tend to be less reliable overall, have lower yields, and tend to be more susceptible to disease when compared to named varieties.
Heirloom seeds are not any more or less organic than other seeds. All seeds are organic unless they have been “treated” with a coating, such as a fungicide to prevent rotting in the soil. Both heirloom and non-heirloom seeds can be treated.
If you have space, plant different kinds of each vegetable, both heirloom and named varieties. Each type will have different characteristics, as well as varied resilience to different insects and diseases. By planting several kinds, you can hedge your bets against which diseases will come around during the growing season.