Unfortunately, I cannot find anything on the specific thickness of the thermal pad, but I did find an article on finding the best fit for your GPU. I'll link it [link|https://whatsabyte.com/9-commonly-asked-thermal-pads-questions-explained/|here|new_window=true] for your viewing, but I'll give you the spark-notes of it.
Ideally, you should use the thinnest thermal pad that you can with the device. Start with a 2mm thermal pad, and if it has an indentation from the heatsink go down a size of thickness. The material that you use also matters, as some materials dissipate heat better than others.