While gestures aren't part of the spell's magic, they might still be interpreted.
Before I start, the spell does explicitly say that the DM determines how the target behaves, so it's ultimately the DM's call. As I read it, gestures are not part of the magic of the spell, but they might be part of the mundane context of the command.
First, spells only do exactly what they say they do, and no more. If gestures were part of the magical command, they would have been included in the text of the spell. Specifically, the one-word limit reduces the spell's versatility--adding gestures basically bypasses that limitation.
Second, the spell only requires that you can see the target, not that the target can see you. If you try to use gestures to Command a creature that can't see you, does it still work? Nothing in the spell text says that the creature can somehow understand a gesture it can't see, because gestures are not part of the spell.
However, creatures have to use mundane, nonmagical context to interpret the command that they're given. For example, "approach" means that the creature has to locate you and figure out how they're going to move closer to you, and "drop" means the creature has to identify whatever they're holding and drop it.
If you issue the command "exit," and point, the creature may use the information you give it in order to carry out its command. However, they are not magically compelled to do anything other than strictly follow the single word command, so they might exit via other means, like teleporting away.