For a general vertical upward throwing motion, the highest point is reached when the velocity is 0. We have:

$$
S=\frac{v^{2}}{2g},t=\frac{v}{g}
$$

Let the factors of velocity and gravity be `a` and `b` respectively:

$$
v'=av,g'=bg
$$

Now we need the height to be the same and the time to expand `n` times

$$
S'=\frac{v'^{2}}{2g'}=\frac{a^{2}v^{2}}{2bg}=S=\frac{v^{2}}{2g}
$$
$$
t'=\frac{v'}{g'}=\frac{av}{bg}=n·t=n·\frac{v}{g}
$$
So we get：
$$
a^{2}=b
$$
$$
a=n·b
$$
And:
$$
a=\frac{1}{n},b=\frac{1}{n^{2}}(a,b,n,g≠0)
$$

C# code:

    using UnityEngine;

    public class UnscaledObject : MonoBehaviour
    {
        private Rigidbody2D rigidbody;
        private float factorA =1f;
    }
    private void Start()
    {
        rigidbody = GetComponent<Rigidbody2D>();
    }
    private void SetTimeScale(float n)
    {
        factorA = 1/n;
        var factorB = factorA*factorA
        rigidbody.gravityScale = factorB;
    }
    private void SetVelocity(Vector2 v)
    {
        rigidbody.velocity = factorA * v;
    }