The bottom line is this, if the prospects don't even get a sniff from their respective countries for the WJC, there are better options than them,
I agree that for a 1st round pick, we should hope that they are able to get an invite from their respective countries for the WJC when they are eligible. The Tyler Seguin rebuttal was weak because Seguin had already made the NHL before his eligibility ran out. Also, he got a "sniff" with an invite before even he was drafted, so that invite basically kills that argument. But I think there are three arguments that can be made for Gaunce having higher potential despite not being invited to the WJC camp.
First and the obvious one is that he has one more year to play or get an invite. Corrado did not get an invite last season - which is equivalent to Gaunce's current season - but got an invite in his final year of eligibility. Gaunce's future with the WJC is still very much open - he has another shot, so you can't write him off based on the WJC not giving him a "sniff".
Second is that you have to remember that competition for a WJC roster spot is a lot higher than other years due to the lockout. For one, RNH is already an NHLer so he would not have been on the team. In fact, all 3 centers on the top 9 (RNH, Strome, Jenner) and 2 wingers who play center (Huberdeau, Schiefele) could have been in the NHL had there been a season. That's 3 slots down the middle that could potentially be open and if there hadn't been a lockout, Gaunce may have gotten a "sniff".
As for the draft-eligibles who had gotten a "sniff", they are considered lottery picks in a very deep draft year and are already dominating their respective leagues. Mackinnon, as we all know has been compared to Crosby. Drouin is showing why he deserves to be on the team. Monahan is also a lottery pick with a high ceiling. These are potential future superstars that Gaunce will not compete against - he's not good enough, and there's nothing wrong with that because we picked him up at 26. His spot would have been where Jenner is playing, or Mackinnon is filling now because of the depth on the team - the bottom 6. The fact that these exceptional players got a "sniff' while Gaunce didn't does not weaken Gaunce's value - because these guys are not Gaunce's competition. There's a reason why they're top 5 picks, and he is a low 1st rounder.
The last argument is that WJC is not the best indicator of success in the NHL. Some go on to have a great career - others don't. Similarly, some who don't get calls from WJC go on to have great careers. So I fail to see why you put so much emphasis on getting "sniffs" from WJC selecting scouts. I mean, just go take a look at what was probably Canada's best WJC squad ever from 2005 when there was the last lockout - sure, there are future hall-of-famers, and guys wearing letters for their team - but there are also 8-9 of 22 guys who never made a name for themselves at the NHL level. That is an overwhelming number of busts in a year that was supposed to be stacked.
Of those who failed to make a name for themselves? They were the guys who made enough of an impression to the WJC guys to be on the bottom 6 on that team - the role that Gaunce would play. Guys like Maxim Lapierre, Travis Zajac, Wojtek Wolski, Devon Dubnyk, Mike Green, Kris Versteeg, Dave Bolland, Tyler Kennedy and Torrey Mitchell were all available for the tournament, but didn't get a "sniff". Yet, these names are more familiar than those 8-9 guys who did not have memorable stints in the NHL.
Of those above, Dave Bolland did go on to make the 2006 squad in his final year eligible and won gold. Gaunce still has that chance. So just because he didn't make it on a talent-loaded lockout year, doesn't mean he is a bad prospect. And on top of that, even if he doesn't make the WJC next season, he can still go on to have a better NHL career than those who did - because WJC isn't a reliable indicator for NHL success.
That's why so many people have an issue with what you are saying. I hope that Gaunce would be recognized and get an invite from WJC next season, but it's not the end of the world if he doesn't, and it certainly should not be the sole reason and basis for writing off a prospect - because it is a poor indicator of NHL success as history has shown.