GPG is probably not the best choice for IM because it lacks Perfect Forward Secrecy. Meaning if someone obtains the key they can read all consent ever encrypted with it both past and present.
PFS requires that both endpoints communicate together so for email's "fire and forget" structure it didn't make sense. But for IM it sure does.
note that with email, you can still choose to advance the ratchet every time a round trip happens by pure coincidence or whatever.
There's also deniable authentication: in the case of email, a simple pre-shared key (instead of public/private) means that the recipient of an email can't prove they didn't forge it.
Of course it does. The attacker just has to steal your token and pin. Because it's unique (well it should be if you generated the key on token as you should have) it's not possible to do so without the target's knowledge. But you can still decypher all past intercepts with it.
Also, some types of tokens like the original openpgp card don't have touch to sign functionality. So it's possible to 'milk' them for decrypts though the gpg agent. This is why I only use Yubikeys now that do have this functionality (though by default it's off!)
PFS requires that both endpoints communicate together so for email's "fire and forget" structure it didn't make sense. But for IM it sure does.