Maybe that the color difference between both gradient stops is to close, so could banding occur. In Photoshop someone would add noise in such a case to avoid banding.
Maybe you did nothing wrong.
And your display is showing the gradient not good enough.
Check in Acrobat Pro what kind of object this gradient is.
Like that with the Object Inspector:
The inspected gradient is a filled path with axial shading.
If it will print alright without banding is up to the PDF workflow system and the plate making process (in case of offset printing).
Depending on the raster system that is used and the corresponding resolution of the plate imaging (hopefully 2540 dpi or higher).