So playing on battle I stab a GK's horse, it reared, GK then stabbed me while stationary and dealt 50% damage to my full health with 40 body armour, 21 str and 4 IF
Now that's bloody over powered, I can't think of a solution to this, but it needs to be addressed, soon
This is usually because GK are dedicated lancers and have a horrendous amount of wpf/PS to do damage with said lance. It's also a case of the horse probably wasn't entirely stationary. If he lanced you as the horse was rearing, I believe the speed bonus still counts. If the horse was turning and you were moving (I assume something of the sort happened because they will want to get out of there quickly), there's also a speed bonus to take into account. Finally, because they are dedicated, that will be a MW heavy lance that hit you. It packs a punch.
Simply put, if you rear a horse, don't assume the rider is incapable of damaging you, they can, regardless of lancer/1h or whatever. You still need to be able to block.
O, and for a foot soldier, 40 body armour isn't actually an awful lot, it's very average (mail with surcoat for instance), so you've gotta expect to be hurt by something like that.
