Spibby

Turns out we have a lot in common! We both come from cultures that don't have the friendliest of reputations. - Gabby, Dragon Dropped

Canon
Spike has been in the series since the pilot, while not appearing in every episode to date, he’s appeared in many and is considered to be one of the 7 main characters. Gabby was introduced in My Little Pony season 6 episode 19 titled ‘The Fault in Our Cutie Marks’, however Spike did not appear in this episode. It wasn’t until season 9 episode 19 ‘Dragon Dropped’ that they would appear together, being the main characters of the episode, along with Rarity and Twilight Sparkle. In the episode Spike tells Rarity that him and Gabby are pen pals, with Gabby explaining that, “Turns out we have a lot in common! We both come from cultures that don't have the friendliest of reputations.” With Spike adding, “ And we're both in the message-sending business!”. Throughout the episode Gabby and Spike show how close friends they are, Spike explaining that he goes with Gabby daily on her rounds. Twilight even states that they hang out all the time. And as Gabby seemingly lives in both Ponyville and Griffinstone she is able to hang out with spike all the time.

Fanon
Spike and Gabby are both similar ages, as well as both being younger siblings, as Spike is Twilight’s younger brother and Gabby is Gilda’s younger sister. As Gabby has said, they are both almost outcasts from their tribes, so it would make sense for them to form relationships outside of their tribes. One reason the ship is quite popular is that their personalities aren’t quite opposites, however they aren’t the same either, allowing for very unique stories and concepts.