I agree with Dave (Note to David - I'm not trying to start a long discussion on easing - lol!)
It's called easing in other software. But it is part of the motion path submenu. Check this article on animation direction for 360:
https://community.articulate.com/articles/articulate-storyline-360-user-guide-how-to-add-animations#optionsScroll down to the direction portion, and it gives the detail, but basically you can set a motion path to start/stop smoothly, and some subtle control over the speed. Since you want to create a loop, you don't want the slow down. That was added to prevent a sudden jerking feel when an animation starts or stops. Think car accelerating and slowing. You're not instantly going 55/88 mph/kph, you work up and down.
This is probably much more than you wanted for this. :)