xybender saidGMO are defined into 2 categories:
1. Conventional GMO, using Mendellian methods of genetic selection
2. Genetic engineering (using new techniques like gene splicing)
This is a definition adopted by many governments (i.e. Canadian).
Even with conventional GMO, foods can loose genetic diversity and can be the source of food allergy when micronutrients are squeezed out of species in favor of selection for bigger carb content, i.e. corn, or wheat, and rice.
With genetic splicing, they inject pig genes, or other animal genes, into fruits and vegetables, just to see if they can make stronger, for example, a tomatoes that can stay on a super market shelve longer.
While fun and novel, genetic splicing produce can cause violent reactions in some consumers.
Unless you eat food directly from the wild by foraging and hunting you've just proven yourself to be a hypocrite. Mendellian genetics of selective breeding for hardier plants and animals has been going on since humans began farming. Select seeding from good crops is exactly what Mendellian genetics is, finding traits deemed desirable and collecting seeds only from that yield for the next season. What I'm trying to say here is that just about everything you buy from the store is a product of selective culturing and breeding whether it be in a short time span or hundreds of years. By the definition you just provided I'd say a good majority of produce at your local market with the exception of wild caught fish would qualify as GMO.
I grow 1/4 of my produce from my yard, herbs, onion, tomatoes, etc. Raspberries grow wild like weeds - their leaves and stems make great tea for male tonic, and they have little spikes that keeps slugs away. Grapes leaves also add a lot to a diet - they also grow 20-30 feet of edible leaves/stems per year.
It's super healthy and possible to go back to a small-farm scenarios that is available to many city dweller. You can grow a pot of green onion, and they will keep coming back every year. They survive frost to 22 degree F. The list goes on and on for the road to self sustainability, together with other small farmers, to share and xchange produce, heirloom seeds, wild seeds.
I am aware of the self sustainable growing methods, wild grape leaves can be pickled and taste great with rice, wild leeks make a great soup, raspberries make a great tea etc. However, these sort discussions about harmful inclusion into the ecosystem can go on forever and in the end it's very difficult to find someone who truly lives by what they preach.
For instance you mention you grow tomatoes amongst other plants which are very likely to not be native to your region. Non-native plants have the potential to become invasive (like the franken-fish) and growing them damages natural ecosystems. Even if there isn't a chance of them being invasive your plants still occupy space and utilize precious nutrients which play just as much a concern in this issue as genetic advantage. That's how nit-picky we can get about these things. If you were to grow only native vegetation to sustain yourself you'd be doing your yard and our ecosystem a favor.
The only truly "natural" method is to mimic the natural ecosystem in your own yard and encourage it. It's a tough game, but it is possible to play along and some are making the effort.
It took 50 years to completely destroy the small farm live style in the US and many places in the world. It will take 50 years to go back there. But it's easier and easier for me. Nuts and berries are rarely genetically selected (almond is not a nut and highly genetically selected). Sprouts like pea sprouts, seeds sprouts, micro greens are easily made just by soaking with water. Growing organic peas is easy.
Most herbs are wild. Most onions are heirlooms as they are highly toxic to insects and have no need to be genetically selected. The book of wild edibles in California is surprisingly large with selection with amazing health benefits, i.e. horseradish, an amazing health booster, stopping cancerous cells upon contact (many publications on PubMed).
Fruits like persimmons are highly nutritious and not selected. Some have been genetically selected recently to remove seeds. This practice of removing seeds is an example of a stupid conventional GMO. Grapes with seeds are more tasty, i.e. the green grape Niagara grape has seeds with complex oil. When blended the whole grape, the seed oil, the grape skin, and the juice make a thick juice that has hundreds and hundreds of nutrients from carb, to protein, to several lipids (fat), flavornoids, and compounds like resvaretrol, an anti fungus and anti aging molecule. Grape seed oil and grape seed extract are being marketed for numerous health benefits.
Similarly, watermelon seeds are highly nutritious and should not be genetically selected out, even as a conventional GMO.