> I wonder does that mean Gab is able to use and benefit from any apps/tools created for Mastodon instances
Theoretically, yes. If Gab becomes an ActivityPub instance, and you have an account on Gab, you should be able to use any ActivityPub client software to connect to it. Since all ActivityPub instances are supposed to implement the same API, clients are essentially agnostic as to which particular instance they're connecting to. It's kind of like how you can use any mail reader to connect to any IMAP/POP email account.
What makes the de-federating thing I mentioned above significant here is that, just because you could use an ActivityPub client to connect to Gab, that doesn't necessarily mean you'd get access to anything in the fediverse outside of Gab. You can only connect to resources outside your instance if your instance is federated with that other one. If other instances refuse to federate with Gab, Gab users would be able to talk with each other and no one else.
Theoretically, yes. If Gab becomes an ActivityPub instance, and you have an account on Gab, you should be able to use any ActivityPub client software to connect to it. Since all ActivityPub instances are supposed to implement the same API, clients are essentially agnostic as to which particular instance they're connecting to. It's kind of like how you can use any mail reader to connect to any IMAP/POP email account.
What makes the de-federating thing I mentioned above significant here is that, just because you could use an ActivityPub client to connect to Gab, that doesn't necessarily mean you'd get access to anything in the fediverse outside of Gab. You can only connect to resources outside your instance if your instance is federated with that other one. If other instances refuse to federate with Gab, Gab users would be able to talk with each other and no one else.