Story time - but in short, no, Gab is not decentralized. They started gab.ai back in the day by forking the Mastodon codebase and changing it to suit their own needs, and they did initially federate with the other Mastodon/Gnu Social instances. The Mastodon devs (and big chunks of the "fediverse" as a whole) are a bunch of totalitarian commies so a lot of them did block or stop federating with Gab. But there is a large (and rapidly growing) chunk of instances that are very free-speech focused who did federate. Later on, Pleroma entered the scene and provided an alternative to Mastodon that had sane devs who were more focused on free speech than on blocking it.. Pleroma uses the same federation protocol as the others (Activitypub). At some point, the Gab developers made some massive changes to the site that completely broke federation. Torba didn't care and just told them to leave it, probably hoping people would join Gab instead and give him more money, I dunno. The end result is that Gab no longer federates with other Activitypub-based nodes and it doesn't look like that will change any time soon. Torba is creating his own walled garden at this point.