Okay. Here's what I'm gonna do. I'm going to take this the only way I can, as an attempt at poetry.
As an attempt at poetry, it is poo.
Gargantuan is a great word, but to use as contrast to a pretty flower is kinda like saying 'supple dump'. Wait, supple dump is actually pretty smooth. Gargantuan lily is like saying humongous nads. No, no, that's pretty good, too. Why are you so bad at poetry? It's fuckin' easy.
I guess the problem is with the image. How can I even picture gargantuan lilies? What's my context, my framework? And if they're beyond a window, how much of them can I really see? If I was small, for example, I'd get a terrible angle looking through a window. Though I suppose I could be a mite on the sill. See, even 'mite on the sill' is badass. I basically crap this stuff.
Taking a lily, a powerful and symbolic flower, and making it large, is not the worst idea as poetic device goes. It could be effective in making the symbol large and increase its importance. It's just that the word gargantuan makes me think of huge spiders, or large nose moles, or the level of appetite required to eat a whole ass.
Short Answer: And 'Beyond the Window' sounds like the secondary title of a bad Lifetime movie. The True Story of Judith Light: Beyond the Window.