The sales analogy is a terrible one-- The only goal is generating sales, and there is no other way to be effective at it, so of course someone who doesn't generate sales will be useless. Scoring goals is not the only goal nor is it the only way to contribute in hockey. It's an analogy without a genuine comparable for what is being argued in the first place. Yeah, if Gaunce didn't produce and also didn't do anything else that mattered, of course he would be useless. That doesn't tell us anything. Hypothetically, if there were some weird specialization that allowed a salesman to increase the sales of his colleagues, or prevented the competition from making sales, it would be stupid not to value that due to lack of individual sales.
Nobody has argued that Gaunce is good offensively (and this specific argument is assuming that Gaunce hypothetically has zero offense anyways), so I'm not sure why that point was made.
Sure, it's pretty much an impossibility for the best forward in hockey to not be offensively productive (because the best forwards tend to tilt the ice so heavily that they inherently do more than simply break even against the best competition), but what does that have to do with anything? You don't need to be the best foward in hockey to be effective and valuable to a hockey team, and it's pretty much impossible to build a team with four lines of three players who can each individually do that, so I'm not sure why that's relevant. I will grant you that if it were possible to build that team (where even the fourth liners could break even against the opposition's best players while chipping in offensively), then sure, Gaunce would be a pointless asset. But realistically, that's never going to be the case.
The only thing that should matter is what degree a player contributes to a team letting in fewer goals than it scores and against what level of competition. If Gaunce is able to be a top-end defensive forward who barely lets in goals when on the ice against top-end competition, he's inevitably going to be more effective and valuable at helping a team win hockey games than most strong, two-way third or fourth liners who can chip offensively, even if he produces nothing offensively. Even the most stacked teams who win cups don't have three players more effective than that on their fourth line. If you had the opportunity to add a version of Sammy Pahlsson in his prime that produced zero points rather than the 5 goals and 20 points he usually does, you'd be crazy not to welcome the addition.
The only question and reason for skepticism should be whether or not Gaunce can even be that player to begin with. Whether or not he can be good enough defensively to compensate for that lack of offense, or whether or not he can contribute modest enough offense that he wouldn't need to be. There are many posters who have legitimate concerns about his ability to do that (mostly concerns about the mobility/quick reactions/assertiveness that is necessary in a great defensive player), and I think that's a very respectable opinion.
However, this idea that he needs to check off both boxes and meet a baseline standard both offensively and defensively in order to be worthwhile regardless of how strong he is at one or the other is really archaic, silly, and illogical. It's manufacturing a bottom line that doesn't have any reason to exist.