The trailer started off with some old scenes to set the context for what it showed, and it gave the implication that Gareth would be joining the survivors on their way to Washington D.C. on their search for a cure for the outbreak. The video is even edited to show a discussion going on in the church which makes it seem like they are already friendly.
In addition, another edit made it look like Rick was talking about Gareth when he said he “doesn’t trust this guy”, but now it looks like it was actually about Father Gabriel.
It was also implied in the trailer that the group that left for Washington did that against the wishes of everyone else, but that doesn’t seem to be the case. The story about the three missing survivors was referenced as well.
Of course, since the show is based on comic books, those who’ve read the books will probably already have some idea about where the plot is going. However, the writers don’t necessarily have to stick to the plot of the books 100%, and they are open to making some alterations to the story here and there. So just because someone is familiar with the storyline in the comic books doesn’t mean that they have reliable information about the new season’s actual plot.