Lae'Zel Eldritch Knight/War Cleric Thrower (HM)

Made it to Act 3 -- Went to equip Kethric's chest piece (Heavy Armor) and it says I'm not proficient.

Until now I used medium armor and never really wanted/needed to wear Heavy. I noticed the racial martial prodigy passive (GIthyanki) and am wondering if the only way around this is the use a heavy armor feat.

I've been following this guide https://www.youtube.com/watch?v=OhL_PvKULYY and maybe I over looked this? The end game gear piece emphasizes heavy armor but the guide doesnt mention this passive or the feat.

Genuinely confused.