Export resources written in GDScript to your C# script with the built-in attribute, by default the [Export(PropertyHint.ResourceType, "...")] will still show all type that can be casted to Resources. This is especially troublesome when you want to export a resources that are written on GDScript to your C# script, and what this script does. Is that it creates a clone property of the original that actually use this attribute and will refer to the correct type.
GDScript.Resources.to.CSharp.Export.mp4
Your script needs to have the [Tool] attribute that derives from ResourcesTypeExportWrapper. This ResourcesTypeExportWrapper derives from Node for the purpose of this examples. But you can change it to any type you want. Then you can use the [Export(PropertyHint.ResourceType, "...")] to your field/property that you want to have the resources on.
This example only provides you the means to export resources written in GDScript to C#. Please refer to the Cross-language scripting documentation if you want to read more about cross scripting.