As reported earlier, if Gambit is 0/0/x, he does not block powers.
However, in the simulator round 1, on the Wolverine essential node; I had Rogue fire ‘gloves off’ together with Gambit spamming his cards. So, it does seem bugged if Gambit is on opponent side.
None of my characters, as far as I know, would be able to stun Gambit.
[Marking as solved. -Dayv]
Isn’t Rogue’s Gloves Off both green, and half passive? Also, I beat Gambit/Rogue/Teen Jean 6x strictly by using Coulson/G4mora with Mystique and perma-stunning Gambit.
I thought I checked the color of it when I saw it go off? But yeah, checking now it is Green. Maybe it changes color when activated?
Never mind then, I was wrong and Gambit is not bugged. 
Well, it does change color when activated, to show you what color is being siphoned from you. But Power Siphon is a passive, so Gambit can’t stop it from firing.
She must have shown red, red, yellow when you checked her during the fight, and the AP cost made you forget it was a passive.