@Dietmar
Try merging your two gradient layers (radial and bilinear) and then applying a large-radius blur eg. Filters>Blur>Gaussian Blur with radius 50px or even 100px.
I think the problem is that you are hitting the limits of what is possible with 8-bits-per-channel graphics.
Your bilinear gradient goes from white to black in about 320 pixels vertically; since 320 is not a multiple of 256, some values are present more than others, eg. there may be two rows with color #808080 but only one row with color #7f7f7f.
Meanwhile your radial gradient goes from white to black in about 1000 pixels, so there may be four rows or columns with color #808080 but only three with #7f7f7f.
Individually these gradients appear perfectly smooth, but when overlaid, where the gradients meet at an angle the slight irregularities in the gradients combine to produce a noticeable stepping.
Supersampling will reduce the undesirable effect but cannot eliminate it altogether.
You can see the problem and the difference that blurring makes in the histogram in Colors>Curves:
..histogram of the radial gradient (nice and smooth)
![Image](http://gimpchat.com/files/4061_dietmar_bg_radial_hist.png)
..histogram of the bilinear gradient (small peaks - some values present more than others)
![Image](http://gimpchat.com/files/4061_dietmar_bg_bilinear_hist.png)
..histogram of the two gradients merged (some nasty peaks)
![Image](http://gimpchat.com/files/4061_dietmar_bg_merged_hist.png)
..after applying 50px Gaussian Blur (much better)
![Image](http://gimpchat.com/files/4061_dietmar_bg_blur50_hist.png)
..and after applying 100px Gaussian Blur (also much better)
![Image](http://gimpchat.com/files/4061_dietmar_bg_blur100_hist.png)