JulieD is right, for this context. In a more general sense, "too far gone" essentially means that something can't be returned to its former condition. The phrase is only used metaphorically, not for speaking about actual distances in space. You could say that someone with a drug addiction is "too far gone" to be rehabilitated. In a cartoon, perhaps, an "evil" character could be "too far gone" for the good characters to reform/redeem him. It's pretty much always negative, but it can be used humorously for talking about something like an obsession. I can't think of any great examples, but maybe something like, "He'll never break up with her. He's too far gone."