[Xubera]

I use this

local frame = CreateFrame("Button")

--[[frame definitions here]]--

frame.leftGrad = frame:CreateTexture(nil, "BORDER")
frame.leftGrad:SetWidth(16)
frame.leftGrad:SetHeight(14)
frame.leftGrad:SetPoint("LEFT", 0, -5)
frame.leftGrad:SetTexture(1,0,0,1)
frame.leftGrad:SetGradientAlpha("Horizontal", 0, 0, 0, 0.2, 0, 0, 0, 1)

frame.rightGrad = frame:CreateTexture(nil, "BORDER")
frame.rightGrad:SetWidth(16)
frame.rightGrad:SetHeight(14)
frame.rightGrad:SetPoint("RIGHT", 0, -5)
frame.rightGrad:SetTexture(0,1,0,1)
frame.rightGrad:SetGradientAlpha("Horizontal", 0, 0, 0, 1, 0, 0, 0, 0.2)

this makes a 32x14 black gradient that starts from transparent to solid to transparent.