Since you didn't provide a screenshot, I used your code to reproduce your GridView. From what I could tell using your code, the gray you're referring to is actually the GridLines.
You can change the visibility or the brush of the lines to meet your needs using these three properties:
I've altered your code to change the color of the GridLines, attached is a screenshot and here is the code:
<!-- stuff -->
If this doesn't help, please reply back with a screenshot of what you're seeing and call out what you're trying to change.
Thank you for contacting Support and for choosing Telerik by Progress.
Lance | Tech Support Engineer, Sr.
Telerik by Progress
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