So it does look like GIs cost guard meter.
Not the... uhh... what-used-to-be-Brave-Edge actual meter, but the flashing indicator around the life gauge.
At ~4:29 Mitsu is in the yellow and performs a GI which whiffs and his guard meter goes from yellow to red. I guess they wanted some cost to be associated with GIs, and I'd much rather have it be linked to the guard breaking mechanic that the meter used to perform supers or EX moves or whatever.
Now I can, in essence, choose to give up a little "guaranteed defense" in being able to block an attack for the opportunity to GI someone and gain the upper hand at the cost of a bit of guard meter. Coolio.
Edit
I'd like to also add that he spams a few GIs when he's in the red which leads me to believe GI'ing can drop the amount of guard meter, but can't break you.
(arbitrary numbers for this example's sake)
You have 100/100 guard meter.
GI'ing costs 10.
Mitsu is at let's say, 13/100 at 4:26.
At anything under 10/100 the status goes to red to warn the player. (which it does in the video)
He GI's (-10) and goes the red state.
He then, since he has nothing to lose at being so critically low, throws out more GIs in desperation (-10, and -10 again).
He can't break himself, but he also can't go below 1/100.
SPECULATION! HA HA!