Everything has a speed, and a movement pool specified in movement points. When the movement pool reaches 0 or less than that, their turn ends, and it moves onto the next creature.
Each action takes some amount of movement points, which are then subtracted from the movement pool of a creature. While the creature still has movement points, it can continue to take actions.
At the beginning of each turn, a creature has it’s speed added to it’s movement pool. If it’s movement pool is still negative or 0, it’s turn is skipped.
So, in your scenario, both creatures have a speed of 100, and a movement pool of 100.
The monster moving 1 tile costs 100 * 13 movement points, and you firing and aiming the gun costs 100 * 10 movement points.
You fire the gun on your first turn, and the monster moves one tile on it’s first turn.
You then have 9 turns skipped while you recover your movement points, and shoot the monster. The monster monster also skips these nine turns, and you can take another action while the monster is still 3 turns in the hole.
In total, it will take the monster 130 turns to reach you, meaning you can fire your gun 13 times.
If you are faster you can, in extreme cases, perform multiple actions per turn, and in less extreme ones, perform multiple actions more quickly, because you both go less negative from performing an action, and recover more quickly.