I have experienced an issue when Gui size didn’t work properly. This issue mostly like visible for mobile devices. I just want to fix it but i don’t know how.
You can try changing the SizingMode to FixedSize, and then change the CanvasSize that fits the size of the Parent. Moreover, you can try what Kir1ll_K said, changing the AnchorPoint to 0.5, 0.5. Make sure that your SurfaceGUI Elements are set to the same AnchorPoint value, otherwise, it may cause problems.