Unfortunately I don't have an environment with skype for business + web gateway handy hence I cannot give you a working rule set or similar.
However, from my experience I am very uncertain how reliable the bandwidth control would work for skype since skype works very unreliable with proxy servers in general.
But if I would have to give it a try, I would do it the following way:
Define a root class with a min bandwidth of 0 and a max bandwidth of the actual available bandwidth.
Define a child class called "Skype", with "Root" as the parent class and the expected bandwidth values.
Create a new rule set called "Bandwidth Control"
Create a new rule inside the rule set called "Bandwidth Control: Skype" as follow Criteria: (use the criteria you are using to detect skype traffic, e.g. if you have a designated port, or a list of URLs) Action: Stop Rule Set (or Stop Cycle if nothing else should be done) Events: Bandwidth.FromServer("Skype")