We have a large tent for a garage, which came with a similar metal frame - fairly robust pipes. What we did was: Spread a 4" layer of gravel for the floor, then lay a pressure-treated 6x6 beam dug in across the front end, where there is no metal bar, and attach the front upright corner posts to that. Then drive T-bar fence-posts next to every other upright post on both sides about 3' into the ground (That's not easy!) and bolt them to the uprights. The rear wall of the tent is attached with lumber to the side of a building, to make a continuous structure.
That thing's 30'x20'x14', and it's not going anywhere!
You should be able to secure your little greenhouse quite cheaply. I'd probably go with a complete under-frame of cedar timbers and attach the metal tubing with sturdy brackets or metal straps. If you're careful with the angles, the whole thing will remain four-square - which is important, right?
