VIM computes the width and height of its display area using columns and lines, not pixels. Resize your GVIM window using your mouse, and you will notice the change.
Let's say the titlebar's height is 40px; when maximized, the width and height of GVIM window is 1024 X 722. Then, the total display area of VIM SHOULD be 1024 X 682 (722 - 40).
If the height of each line is 20px and each character (column) takes 5px, then there will be 51 (1024 / 20) lines and 4 (1024 mod 20) pixels, 136 (682 / 5) columns and 2 (682 mod 5) pixels.
The actual width and height of VIM's display area is 1020 X 720 while the resolution of the GVIM window is 1024 X 722, this making the rest area unused (the gray area).
How to fix it?
Never maximize the GVIM window, or you can use this trick: