  We show that if the Gabor system $\{ g(x-t) e^{2\pi i s x}\}$, $t \in T$, $s
\in S$, is an orthonormal basis in $L^2(\mathbb{R})$ and if the window function
$g$ is compactly supported, then both the time shift set $T$ and the frequency
shift set $S$ must be periodic. To prove this we establish a necessary
functional tiling type condition for Gabor orthonormal bases which may be of
independent interest.
