They act as polearms and would stop horses, but you need a 140+ weapon length polearm to rear the horses. Non of the greatswords have 140+ weapon length.
Polearms used with no shield get -20ish or so length, 2h swords get +50, so a greatsword is longer ranged then a glaive (which is barely long enough if the lancer is stupid and too short if the lancer is not stupid, granted you can evade and slash, and is also the longest polearm you can swing sideways).
Of course, you could use a spear with a shield and outrange them. But then, amazingly, you have no slots for another polearm. So either you pick a polearm which can stop horses and support or you pick a polearm which is great for general fighting, but you can't take both. The argument that all polearms can stop horses, well, ye, but if someone is daft enough to run into a poleaxe stab, then you can halfsword your greatsword if for some reason you want to rear the horse instead of stabbing it in the face.
