how do i make this move slowly over time

i got this problem where i have my GUI labels and i want to as soon as you hit new game, the menu will slide off to the left wich is -600 but i simply cannot find a way to do it :confused:
any wise heads out there? thanks alot in advance!

void OnGUI() {
	GUI.skin = menu_styles;
	if (main == true){
		GUI.BeginGroup(new Rect(placeing, 0, 800,900));
			if (GUI.Button(new Rect(0, 40, 512, 128),"",menu_styles.customStyles[0])){
					//main = false;
					//placeing = (-600f) * Time.deltaTime;
					placeing = BLANK
				Debug.Log ("lol");
			}
			
			if (GUI.Button(new Rect(0, 170, 512, 128),"",menu_styles.customStyles[1])){
				
			}
			
			if (GUI.Button(new Rect(0, 300, 512, 128),"",menu_styles.customStyles[2])){
				
			}
			
			if (GUI.Button(new Rect(0, 430, 512, 128),"",menu_styles.customStyles[3])){
				
			}
		GUI.EndGroup();
	}
}

You could have a destX variable holding the desired X coordinate and make placeing moves gradually to it:

public float speed = 800; // speed in pixels/second
public float destX = 0; // set the desired X coordinate here

void Update(){
    // slide placeing towards destX at Update
    placeing = Mathf.MoveTowards(placeing, destX, speed * Time.deltaTime);
    ...
}

void OnGUI() {
    GUI.skin = menu_styles;
    if (placeing > -600){ // only draw the GUI when menu not completely hidden
        if (main == true){
            GUI.BeginGroup(new Rect(placeing, 0, 800,900));
            if (GUI.Button(new Rect(0, 40, 512, 128),"",menu_styles.customStyles[0])){
                destX = -600; // sets the new destX to start sliding
            }
            // other buttons
            GUI.EndGroup();
        }
    }
}

Just set destX to 0 to make the menu slide in, and click the button to make it slide out.