All vices are tough to quit, the issue here of course is that some vices are illegal and some are not. What is illegal and what is legal, in some cases, has nothing to do with how harmful it is. It is just a matter of the culture, money/influence and what the laws are at the time.
But in Gordon's case, he cannot smoke weed if he wants to play in the NFL, IF that is what the issue is with him. These days if a person wants the positive effects of cannabis they can take THC free CBD oil. Which is what Gordon should have done if he felt it necessary for his heath.
Mental issues are tough, and the modern day world is extremely difficult for some to endure since so much has changed in our societies so fast without thinking of its effects. We humans make the mistake of forgetting we are animals, with instincts and reactions that are 1000s of years in the making and they are not used to what we bombard ourselves with on a daily basis.
Yet, you have to try and be responsible of if your are in positions where people dictate what you can put into your body.
Unfortunately for Gordon, I think this is it for his chances to play in the NFL.