Modified vanilla maps are allowed just as long there's effort put into them. And did you took the textures from SRB2 Heroes? While I like the look of it, I think you should ask and/or give credit before/after you do.
I think the idea is, Michael, that if you're making this a mod for SRB2, you'll have to change the actual map structure yourself in order to give it substantial enough value for it to be placed in releases. If you're making this a mod for a different game, you should ask the person who originally made the Green Flower Zone map for permission to use their work.
Any use of SRB2's original assets outside of SRB2 requires permission from the original author of those assets, or from the dev team itself. This doesn't apply to the code as it is licensed under GPL v2, nor to any assets ripped from other games as we don't own that.
We generally don't give permission for our maps to be used in other projects. If it's just an example map it might be okay, but I'm not the one who can say for certain. (And you could do a lot better for engine example maps than GFZ1 anyway.)