Gabriel hasn't been in his right mind since the dead took over, it was only a matter of time until he completely snapped once he was re-introduced to "good" people. The Alexandrians are a constant reminder of who he turned his back on. As wrong as it is, I was finally interested in what happened to him and I'm not sure if he's going to make it to the end of the season. It was a really good episode emotionally.
The Alexandrians are cowards, and so out of place now compared to the rest of the survivors in TWD world. I don't know how some gang or group hasn't wiped them out, like what happened to Noah's community. I don't doubt that Dianna will believe Gabriel and the Alexandrian that survived the run. I wish they had spent more time building up the dynamics though, showing her being uncomfortable when she sees how hardened the group is. I'm still not invested in the abusive doctor/wife/rick triangle. I wish they had scraped that plot and focused more on letting the Alexandrians realize what it takes to make it now.
They keep killing off the black characters for the same reason they kill the women characters. They're disposable/replaceable. Morgan will be back at some point, and until then the show has poc and minority characters to keep the show "diverse". We had to lose Beth, Noah, Tyrese (and possibly Gabriel) so we can keep Glenn, Tara, Eric and the boyfriend. I don't agree with it, but that's what the show does.