I ever understood nothing in the establishment of the lists of the competitions GP. The national champions are automatically hired in the one who in places in their country?
It is up to the host country which three teams from their own country attend a GP. Usually, this means that--yes--the National Champions are a guaranteed selection. However, Weaver & Poje did not go to Skate Canada in 2016 when Virtue & Moir returned. This year, Weaver & Poje medaled at Worlds and were the top-ranked team at Worlds so they are likely to be Canada's number one home country pick. Gilles & Poirier are a question mark. They finished ahead of Weaver & Poje at Canadian Nationals. If two teams from the same country are ranked in the same group, either the group 1-3 or the group 4-6 at Worlds, both cannot skate at home. The second team usually has to travel. In this case, however, W&P were in a different group of three than G&P. So Canada can choose to invite both teams.
They might. Canada is short on dance teams right now, with Fournier-Beaudry & Sorenson sitting out a season, Lajoie & Lagha possibly staying on the JGP one more season, and Arnold & Williams placing relatively low at 4CCs this year (though I think A&W are still a likely host pick if L&L don't move up and as long as Soucisse & Firus can get two spots elsewhere).
It may also seem safer to invite Gilles & Poirier than Chock & Bates who have defeated Weaver & Poje about 50% of the time over the past 4 seasons. (And as Debbie says, C&B may want to compete later in the GP season if their training is delayed due to injury).
On the other hand, it's also a bit risky for Canada to invite Gilles & Poirier and Weaver & Poje to the same GP. If Canada wants Weaver & Poje to stay in international competition, then they don't need to set up a homefield head-to-head this early in the season and leave one of these teams in a subordinate position so early on. If both teams make the GPF, it will happen soon enough. When two teams are ranked very close, it's common for countries to send out the second team. Weaver & Poje and Gilles & Poirier finished back to back at the Olympics this year. It's also often better for the second team to go out and prove themselves against a #1 ranked home team by traveling and earning their way into the GPF. But . . . Gilles & Poirier's results in away competitions on the GP over the past three seasons haven't actually been as good as their end-of-the season results at Worlds. (They've lost to Sinitsinia & Katsalapov, Stepanova & Bukin, and in 2016-17 to Hubbell & Donohue). While G&P's result at SC in 16-17 was very good. (They finished ahead of Cappellini & Lanotte and Stepanova & Bukin).
So Canada may go with both Canadian teams at home this year. Or they may not.
I wouldn't. But Skate Canada might.