You could use lerp (linear interpolation) for this, but you could also program it yourself
Create a global variable speed
on right arrow down - sprite set x to sprite.x+speed
add a subevent
system compare variable speed is less than 10 - add 1 to speed
on left arrow down - sprite set x to sprite.x+speed
add a subevent
system compare variable speed is more than -10 - subtract 1 from speed
X on right arrow down (so inverted statement)
X on left arrow down (so inverted stament)
subevent
system speed is between -10 and -1 - add 1 to speed
system speed is between 1 and 10 - subtract 1 from speed