What is my Medi-Gel Capacity?

No, the only way to see your medi gel capacity is to have as many medi gels as you can carry...and also know that you're maxed out.

You can take an educated guess however; you start at 3, and each level of Medi Gel Capacity boosts it by one. Additionally there's a quest you can do for Dr. Michel (maybe also doctor Chakwas?) that increases your capacity by one. Therefore, your character in the screenshot must have 5 or 6 max medigel; they started with three and they must own Med Gel Capacity II already. Only you would know whether you got the extra capacity from the quest.

I'm guessing it wasn't a priority to display it in-game because there's not much you can do about it; you can't intentionally fill up on medi gel, and there's no reason to abandon medi gel if you're maxed, since it gives XP. But ME3 is largely missing an "inventory" screen for items like this.