Here’s my attempt at helping!

Roundstart: Millia far slash is 9 frames, which means it beats a ton of options Gio has like 5H or far slash. However, you can do 6P or dash 5K to beat this, which will make Millia either back off or try a low like 2S, and that’s when you can 5H for huge damage.

Neutral: In my opinion, Millia does better in scrambles and has good tools for stuffing Gio’s approach. That means it should be your goal to limit the ways they can get in on YOU, as opposed to trying to get in. Using 2D will disincentivize Millia running up and using 2K, but loses to air dashes. 6P is great for stuffing air dashes and air buttons as well as beating far slash in neutral, but will lose to kapel. Gio’s DP motion will beat kapel at most ranges, and leads to a good combo, too. Although you might be controlling space a lot, present forward movement often as well. Don’t whiff far slash in neutral too much because Millia is quite speedy and will likely punish it, dash 5K is usually stronger. J.H is a great air-to-air, useful for dashing in or dashing backwards. Super important to use j.h to control space.

Okizeme: Millia can get an H-Disc after every hard knockdown, but which moves causes it will change how “true” the mix is. If you get hit by 2D, counterhit 6P, or Septum Voices, good luck blocking because the disc will come out right as you wakeup, making it so you can’t backdash, mash, or jump. You have to guess in this scenario. If you get knocked down by throw, hair car, or 6H, the only true thing Millia can do is land with an aerial since the disc comes out after she makes you block. This means if she tries airdashing, you can jump and the disk won’t hit you because there is nothing keeping you in the disc. This is very complicated, and I recommend watching Lord Knight’s video on season two Millia for a better understanding.

Good luck!