JumpAnimationSystem.cs

using UnityEngine;
using Unity.Entities;

public class JumpAnimationSystem : ComponentSystem
{
    struct Group
    {
        public Rigidbody2D rigidbody2ds;
        public Animator animator;
    }

    protected override void OnUpdate()
    {
        var group = GetEntities<Group>()[0];
        var velocity = group.rigidbody2ds.velocity.y;
        group.animator.SetFloat("velocity", velocity);
    }
}