> I have tested your APK on my Huawei Honor 5X of 1080X1920 resolution (16:9 1080p) and couldn't see the gray bar.
My assumption is that the the devices you have the issue on are not using this resolution and ratio.
Please refer to our tutorial on handling multiple screen sizes.
The gray color also comes from the fact the first/background layer in your layouts is transparent.
Set it to not be transparent and to display the black color for example, the border will still display but in a color that is more appropriate to your game's color theme.
for now, only that ...