As far as I know, you can't use gradient on the backdrop so you can't use a backdrop to tile your texture if that's what you want.
You seem to do some crazy stuff though..
Here's an example of tilling a texture vertically and fading it (just used a random texture from WoW):
lua Code:
local frame = CreateFrame("Frame", nil, UIParent)
frame:SetHeight(512)
frame:SetWidth(256)
frame:SetPoint("CENTER")
-- Instead of using a backdrop, you create a texture as the background
frame.bg = frame:CreateTexture(nil, "BACKGROUND")
frame.bg:SetAllPoints(frame)
frame.bg:SetTexture("Interface/OPTIONSFRAME/NvidiaLogo", true)
frame.bg:SetTexCoord(0, 1, 0, frame:GetHeight()/64)
-- the last bit above is basically <frame-height>/<tex-height>,
-- to squeeze in as many full-sized tiles as possible, so you might
-- have to tweak this to your preference. The original texture I
-- used is 256x64 in size.
frame.bg:SetGradientAlpha("VERTICAL", 1, 1, 1, 0, 1, 1, 1, .6)
Looks like this (again, the skybox):
