I think I understand what you're trying to do but unfortunately - as far as I know, but I'd be happy to be proved wrong - there's no easy way to do what I think you're trying to do in Scribus.
It's possible to create some rectangles with tiny gradients at one end that fade to transparent, with each rectangle having a gradient at a different side to make the three sides of the "box". However, you can only set gradient stops at an integer percentage of the width of the frame so the top/bottom gradients will be longer then those at the sides (see attached image Capture1.JPG, left). And the result doesn't look very nice. (Putting the rectangles on different layers and changing the blend modes of the layers doesn't help much either.)
Another way is to create a line style with a gradated set of lines from black to white at different line widths. Then apply the line style to a rectangle going around the text but (on the top page) reaching further down the page than the text. Then create an "overlay" around the outside of the text with some other rectangles using the path operations (see attached image Capture1.JPG, right, where I've also added a rounded corner). However, this doesn't look very nice at high zooms.
I've attached the file borders.sla so you can see how I've constructed - very quickly and crudely - the referenced image. (Scribus seems to have trouble drawing the gradients properly sometimes so you might need to scroll around randomly selecting things until they draw as shown in my image. It's not something Scribus is brilliant at.)
You could create something better in Inkscape and import the SVG into Scribus but I don't have a lot of confidence that Scribus will be able to display the SVG properly (it can't handle a lot of the transparency stuff well).
Another option would be to create the box in GIMP (quite simple using Stroke on Path) and import the image into a background image frame - see other attached images Capture2.JPG and border.jpg (where I haven't bothered removing the bottom of the box). Exporting at 80% on an A4 page creates a 31KB file that actually looks a bit better than the original at high zooms. (border.jpg might not look good in a web preview but it looks fine in Scribus at Full Resolution preview.)
Other than that, sorry I can't help much more. This sort of thing isn't something that Scribus is built for.