Why I cant access a new attribute I created in a propertydrawer script from mono script?

using UnityEditor;
using UnityEngine;

public class GapRangeDrawer : PropertyDrawer
{
    float minRange;
    float maxRange;
    float sensitivity;

    public GapRangeDrawer(float minRange, float maxRange, float sensitivity)
    {
        this.minRange = minRange;
        this.maxRange = maxRange;
        this.sensitivity = sensitivity;
    }

    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        if (property.propertyType == SerializedPropertyType.Float)
        {
            EditorGUI.BeginProperty(position, label, property);
            EditorGUI.Slider(position, property, minRange, maxRange, label);
            property.floatValue = RoundValue(property.floatValue);
            EditorGUI.EndProperty();
        }
        else
        {
            EditorGUI.LabelField(position, label.text, "Use [GapRangeDrawer] with float values only!");
        }
    }

    private float RoundValue(float value)
    {
        return Mathf.Round(value / sensitivity) * sensitivity;
    }
}

then in a mono script:

public class CubeSpawner : MonoBehaviour
{
    public GameObject cubePrefab;
    [Gap

but GapRangeDrawer is not exist.

i tried to create my own attribute but I can't access it.
i wanted to make a custom range attribute.

the propertydrawer script is under Assets > Editor folder but still I can't access the new attribute.

Apply the CustomPropertyDrawer attribute to your PropertyDrawer class. Make sure this class is defined inside of an Editor folder.

[CustomPropertyDrawer(typeof(GapRangeAttribute))]
public class GapRangeDrawer : PropertyDrawer
{
    ...
}

You also need to create a PropertyAttribute. Unity provides an example further down the Property Drawer Documentation Page.

Your attribute will look something like this.

public class GapRangeAttribute : PropertyAttribute
{
    float minRange;
    float maxRange;
    float sensitivity;

    public GapRangeAttribute(float minRange, float maxRange, float sensitivity)
    {
        this.minRange = minRange;
        this.maxRange = maxRange;
        this.sensitivity = sensitivity;
    }
}

Now you can use it in the way you intended.

[GapRange(0, 100, 3)]
public float someValue;