Gab was an early attempt at "free speech" but got attacked mercilessly since it was so centralized, also there's a lot of drama about the founder and userbase (not surprise there). Mastodon is a specific implementation of the ActivityPub protocol. "Fediverse" refers to the whole family of ActivityPub protocol implementations and includes e.g. Pleroma, Soapbox, Misskey, Friendica, and more. Mastodon is generally run by trannies who want a hugbox and block anyone who e.g. doesn't play the pronoun game (if you call a man who is pretending to be a woman a man and vice versa, you'll get banned/blocked by most Mastodon instances). The other instances don't really block instances unless they post illegal things (and there are some, but the general Fediverse crowd you're seeing today stays away).