Rotating an object on Z axis while moving

Hello, I really don't know what should I do.. I have no more ideas :/ I've already seen some q&a about rotating but "I still haven't found what I'm looking for" :D I'm creating 2D game and now, I'm making object rotations. I think my English is bad so I add a picture for you with that, what I'm thinking about.

Object has to rotate only on Z axis while its moving up or down or sides.

I wrote that, but an object is just making a really silly things when I move - I know that :D .

     public float speed;
     void Update () {
                 float movelr = Input.GetAxis("Horizontal") * speed * Time.deltaTime;
         transform.Translate (movelr, 0, 0); 
         
                 float moveud = Input.GetAxis("Vertical") * speed * Time.deltaTime;
                 
                 transform.Translate (0, moveud, 0);
     
                 if(Input.GetKey(KeyCode.C))
                 {    
                 transform.Rotate(0,0, Time.deltaTime * 180);
                 }
 //and I have gravity, when object is falling I have to rotate him on Z axis, I hope you understand me :/
             if (transform.position.y > -4.5f) 
             {
             Physics.gravity = new Vector3 (0, -4.5f, 0);
             }
     }

I start with unity and C#, please guys, help me :) Thanks