Dva can use  Gladiator’s Medallion while channeling [W] Defense Matrix. Most heroes can’t use Gladiator’s Medallion while channeling an ability.
hmmm, let me think why…maybe because enemies can stun you to cancel your Matrix?
This isn’t necessarily a bug. Some abilities are “soft channels”, where you can still cast skills while using them, except using another ability will cancel the channel. Defense Matrix is included in this, as are Genji’s Deflect and Chen’s trait for example. Since Gladiator’s Medallion has no cast time, it doesn’t actually interrupt the channel when you cast it.
I can’t think of any off the top of my head, but there are other heroes with soft channels and activated skills that you can use without breaking the channel. So this is actually consistent. It’s up to Blizzard to decide if that’s balanced or not, though.
And I did test Gladiator’s Medallion on a few other heroes. You can use it on any hero without breaking a soft channel.
Mei for example can’t use the medallion if she is ice blocked, leaving a small window of exposure where she can be caught in chain-cc.
Dva shouldn’t probably be allowed to use either for balancing reasons. There are many other situations which benefit greatly from being able to the medallion most of which we won’t catch until the patch goes live.