I agree mnawij. While the gradient tools are fine for most needs, when you want to try anything a bit more creative they can get quite awkward to use (and sometimes they just don't work properly).
Your PDF does - as utnik said - look like it's made up of two linear gradients which isn't possible in a single object with Scribus but you can create two shapes (a quarter-circle at the bottom-left with a rectangle underneath, each with its own linear gradient) and overlap them. You would have to play around with the gradient stops and vectors to get them looking just right. (See my attached - quickly and crudely made - example.)
And to add to what utnik said: You can create the radial gradient in 1.4.6 with just three stops, just drag the left-hand stop to the right instead of creating a new one along the gradient. (However, sometimes it can look like the area of the gradient where the stop is is a bit darker then the surrounding colour. I don't know if that shows up on printing, or whether it's my set-ups that are to fault, or just my eyes deceiving me.)