You can't make vim inside the terminal actually full screen, unless you're lucky enough that both the width and the height are multiples of the cell size (plus the fixed UI chrome taken into account).

The terminal is a text grid, and if you force it to a maximized size which doesn't match this grid then there's necessarily a margin which is inaccessible to vim.

You probably don't realize it with xterm because xterm's background color is the same black as vim's background color; whereas gnome-terminal's is different.