If zwet being scum implies Gorrad also being scum but not vice versa, then there must be more possibilities where Gorrad is scum than where zwet is scum, since every possibility where zwet is scum is also a possibility where Gorrad is scum but not vice versa. Therefore Gorrad must be more likely to be scum than zwet.
Or to look at it another way. Assume you think both zwet and Gorrad are likely scum, and want to lynch both unless one is proven town.
If you lynch zwet first...
* If zwet and Gorrad are both town, you lynch two townies.
* If zwet is town and Gorrad is scum, you lynch a townie and a scum.
* If zwet and Gorrad are both scum, you lynch two scum.
If you lynch Gorrad first...
* If zwet and Gorrad are both town, you lynch
one
townie.
* If zwet is town and Gorrad is scum, you lynch a townie and a scum.
* If zwet and Gorrade are both scum, you lynch two scum.
On the other hand, if you don't think they're both scum (you only want to lynch one of them), you should be voting the one who is more likely to be scum. Which is Gorrad.
There is no protown reason to lynch zwet over Gorrad if you really believe that zwet being scum implies Gorrad being scum.