Gems are based on counterparty, while it's based on the bitcoin blockchain, this doesn't mean it's not an altcoin.
To answer the question of whether it's an altcoin, how does one convert gems to bitcoins, and how do one acquire gems using bitcoin? As far as I can tell, the only way to acquire gems is through an exchange, or in exchange for getting ads through your app, and I'm guessing one would need to sell the gems on an exchange to convert to bitcoin (though this isn't clear).
If this is the case, that is solidly in altcoin territory.
You snooze you lose. I posted to help the smart hearers. I'm glad the 'smart Mods' on here don't get.....