... I saw no way to remove the gradient caption background in Juicebox Pro...
To remove the gradient, make sure that both captionBackColor and captionBackTopColor (in JuiceboxBuilder-Pro's 'Customize -> Color' section) are exactly the same (in color and opacity).
Because captionBackTopColor has different default values for Large Screen Mode (rgba(0,0,0,0)) and Small Screen Mode (rgba(0,0,0,0.3)), if you want to ensure that there is no gradient in the caption in both screen modes, set captionBackColor and captionBackTopColor to non-default values (but still both identical to each other).
There really have been a huge number of bugfixes since v1.1.1.
I would certainly recommend using the latest version and trying to fix any problems you have with that than trying to fix problems that you are having with v1.1.1.
Hopefully my notes above will help resolve the problem you're having trying to remove the caption gradient.
If you let me know what other problems you're having with v1.5.0, I'll certainly try to help.