I think its a great topic! I also grow my own veggies and fruit.
Not quite sure about the hardiness, because we don't use that map in Europe.
What you could probably grow:
spinach
perpertual spinach
swiss chard (rainbow coloured!)
red beets (harvest the young greens)
amaranth greens (naturally resistant to Roundup, take that Monsanto!)
quinoa greens
lambs lettuce (great for colder climats)
rocket
bok choy and paksoi
mustard greens or amsoi and kaisoi
kale and borecole
broccoletto (looks a bit like broccoli, but you eat the leaves, not the flower heads)
chinese broccoli or chinese kale
red cabbage, white or green cabbage, savoy cabbage (all take up a lot of space per plant)
brussels sprouts (harvest in winter, but acquired taste)
different types of leaf lettuce
different types of head lettuce (just sow thick and harvest thinings as greens, iceberg and crispleaf are more hardy in cold wheather)
cardoon (a thistle of which you eat the blanched leaves)
mountain spinach (atriplex hortensis, grows like weeds)
oyster leaves, saltwort and land seaweed (with a salty taste)
leeks and spring onions
purslane and miners lettuce
sorrel
endive
radishes (some are grown for only their greens)
peas, use tips and young leaves
Think this will do as starters...
If you want non GMO, look for heirloom seeds.