It's clearly something that adds to assassinate ability where Assassins critical hit foes who haven't acted in the first round.
Bearing in mind assassin gets +d6 sneak attack every second level, Gloomstalker must get something more.
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[Edit] Looked it up myself.
Dread ambusher: level 3 ability, +Wis to initiative, extra attack with +d8 damage.
So level 6 Assassin has +4 initiative, +7 attack, weapon damage +4 dex, and +3d6 sneak attack Level 3/3 Assassin-Gloomstalker has +5 initiative, +6 attack, 2x weapon damage +3 Dex, +d8 dread ambusher, and +2d6 sneak attack.
Considering the multiclass gains on average an extra doubled die on round 1, but falls behind rounds 2+, I think it's pretty balanced. Actually, it's kind of weak once you realise they're missing out on Expertise and Uncanny dodge.
