Depending on the art style and what looks good visually, you could have the torso inherit its rotation from a single IK bone. Then moving the IK handle around would cause the character to rotate up/down to aim toward the IK handle (assuming you aligned the handle that way initially). In GMS2, the goal would be to override control of that IK handle's position programmatically to line up with where you're aiming.
Alternatively (and this is the option that allows for more customized posing of the character from one extreme to another), you could have an animation for aiming up and an animation for aiming down, and blend between the two at runtime to hit the angles inbetween. For this approach, I sometimes also have a middle 'normal' straight ahead aiming animation inserted into the blend mix halfway, if there's any awkwardly interpolated posing that happens when just using up mixed with down.
