It should work as long as you can vent it on sunny days and heat it if it drops to freezing, or below what your plants need.

I think it sounds like a neat idea. Hey, nothing ventured nothing gained right? Your metal gazebo is basically the same as the metal framework that I have in my greenhouse. Use heavy guage plastic sheeting and metal clips to attach your 'glazing' and you should be in bidness.

Be sure to attach the plastic to the base to prevent wind, varmints and bugs from getting inside. If you can run some power out to it, you should have a fan going to keep the air moving inside. Use a power strip so you can plug in grow lights too at the same time. You also might want to run a hose out there so you can keep it moist inside and so you don't have to lug buckets of water.

Those nice powder-coated utility shelves that you can get at the Wally are perfect for greenhouses. Since they are basically made of wire you can hang grow lights from the bottom of the shelves to light seed flats. I used one last Spring and set it up in my kitchen. I used one large drop cloth and clothes pins to secure the plastic. I had all the grow lights attached to the same power strip so I could turn them on and off at one switch. I also put a humidifier under the plastic to keep it moist and the baby plants loved it.

Cheryl