Straight to the questions:
1. I was not able to reproduce an issue related to the implicit style targeted at GridViewCell. I suppose that there is something specific in this use-case and it could be great if you can send us a small application which can help us to see what is wrong.
2. You need to remove the border of GridViewCell in its current state, as it is demonstrated here.
If you need any additional information please let me know.
the Telerik team
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