You definitely don't need Gjallarhorn.
I can only assume they're not very good.
Iam shooting him with Truth, no need for Gjallababy. Basically every maxed RL which has atleast Grenades & Horseshoes perk is enough for Crota. Before I bought Truth from Xur, I did use this:
Without G&H perk its easy to miss Crota (seen many missiles flyby...).