Can an 8ft x 6ft wooden greenhouse be built on a plastic and gravel filled eco base without being secured to the ground?
I wouldn't think so. When the wind hits Gale Force, it is likely to remove your greenhouse. Certainly, anything in my garden that isn't secured just flies off.
Welcome to the forum.
I had an 8x6 greenhouse "secured" to railway sleepers and the whole thing lifted into the air, moved about 5m then landed , re "flatpacking" itself. Don't do it.
In the words of Roy Cropper in Corry: " if you fail to prepare, you should prepare to fail"
the enduring gardener
You have to anchor it down. You could use that for the majority of the base but put some concrete piles at the corners and bolt it there.