For example I want a part to go transparent but only for the player with the gamepass?
Suppose the gamepass is: "www.roblox.com/xyz"
According to the given example, please provide a script for reference and example.
In the same process, explain how I can make and use a client-sided script here.

2 comments
what does the pcall and tostring do?
The pcall will un the function in protected call mode, meaning you can ignore or handle errors during the call. I don't believe it's necessary for this, though. As for the tostring(), that would be unnecessary. If the function pcall calls errors, it would return the error message as the second return value.